The Silent Threat to Patient Safety: Combating Airborne Pathogens by Investigating Natural Ventilation’s Impact on Indoor Air Quality in Hospital Wards

Introduction

Indoor air pollution has become a significant global concern issue, with the World Health Organization (WHO) stating in 2018 that approximately 3.8 million premature deaths per year result from exposure to hazardous pollutants in indoor environments.¹ Numerous studies have shown that airborne microorganisms in hospital environments are major contributors to infections, allergies, and immunotoxic disorders.² A growing concern in healthcare settings is the prevalence of healthcare-associated infections (HCAIs), which primarily result from the transmission of pathogens between patients and healthcare workers via airborne routes and contaminated surfaces.³ Chernet (2020) reported that in developed countries, over 50% of patients in intensive care units (ICUs) and 5% to 15% of hospitalized patients are affected by healthcare-associated infections (HCAIs).⁴ Data from the WHO (2011) reveal that 7–10% of hospitalized patients get at least one healthcare-associated infection (HCAI).⁵ This scenario therefore leading to longer hospital stays, long-term disabilities, and increased costs for both patients and healthcare systems. In this context, enhancing IAQ and ventilation systems is now considered as a key strategy for reducing pathogen transmission.⁶ A study conducted by Basinska et al. (2019) similarly emphasized that ventilation in healthcare facilities is crucial for providing protection against the variability of indoor air contaminants, including physical, chemical, and microbiological factor.⁷ Inadequate ventilation can lead to the accumulation of elevated indoor microbial loads and other pollutants.^8-9

Hospitals use three main types of ventilation, namely natural ventilation, mechanical ventilation, and hybrid systems. Each has its own advantages and challenges that need to be carefully evaluated for optimal implementation. Among them, mechanical systems are still the main focus of ventilation strategies in healthcare facilities¹⁰; however, challenges persist in preventing airborne infections and maintaining optimal indoor air quality. This is supported by the findings of Rautiainen et al. (2019), which found that total volatile organic compound (TVOC) levels in hospitals ranged from 10 to 5660 µg/m³, which is extremely high and much greater than acceptable standards, even with the implementation of HVAC systems.¹¹ Specifically, xylene levels in pathology wards were measured at approximately 3400 µg/m³, indicating that materials utilised in these settings release VOCs at potentially hazardous concentrations.¹¹ Liu et al. (2018) have describe the inherent limitations of mechanical ventilation, stating that HVAC systems create conditions that conducive for microbial growth including optimal temperature, humidity, and nutrient availability within components like air ducts, filters, and heat exchangers.¹² The multiplication of microorganisms under these conditions leads to the formation of bioaerosols or the accumulation of microbial-laden dust, that is then dispersed into indoor spaces through the supply airflow. This makes indoor air even more polluted and poses risks to occupant health. Even more concerning is a study by Chezganova et al. (2021), which revealed that ventilation-associated particulate matter (Vent-PM) can act as a reservoir for multidrug-resistant organisms (MDROs) and healthcare-associated infection (HCAI) pathogens.¹³ Their research revealed that more than 70% of Vent-PM samples were contaminated, with 60% hosting multidrug-resistant bacteria and 48% hosting biofilm-producing strains.¹³ These results underscore the critical need for comprehensive air quality management plans which integrate humidity control and optimized ventilation dynamics to reduce airborne contamination in hospitals. The study by Monge-Barrio (2022) corroborated that the use of natural ventilation systems significantly enhanced indoor air quality (IAQ) and diminished the risk of airborne infections, including respiratory illnesses such as COVID-19.¹⁴ Onmek et al. (2020) discovered that hospital wards that used natural ventilation maintained the levels of microbes below the acceptable limits sets by the National Institute for Occupational Safety and Health (NIOSH) and the World Health Organization (WHO).¹⁵ Specifically, bacterial concentrations were less than 500 CFU/m³, while fungal levels were less than 1000 CFU/m³, indicating effective air quality control. This also aligns with the findings by Morawska et al. (2020), who highlighted that natural ventilation can significantly improve indoor air quality by lowering the concentration of airborne pathogens in the air.¹⁶ This is because natural ventilation allows fresh air to enter indoor spaces, which helps to disseminate and dilute infectious particles, a crucial factor in for stopping the spread of respiratory infections. Furthermore, Liu (2018) emphasized that HVAC systems are associated with a significant increase in Sick Building Syndrome (SBS) morbidity, ranging from 30% to 200%, compared to natural ventilation, This increase is compounded by the association of HVAC systems with multi-chemical sensitivity and building-related illnesses (BRI).¹⁷ Building on these findings, the present study aims to evaluate the impact of natural ventilation on the concentration of airborne microorganisms, alongside its effect on indoor air quality (IAQ) in various hospital wards with various medical disciplines. Hospital wards differ in activities and environmental conditions depending on their medical specialties. Consequently, the microbial load in these wards is expected to vary significantly, as observed by Kayta et al. (2022).¹⁸ Liu & Dickter (2020) also discovered the variations pattern of microbial load, reporting that ICUs and surgical wards have the most cases of nosocomial infections due to their distinct clinical functions.¹⁹ In ICUs, aggressive interventional treatments including invasive catheters, and frequent antibiotic usage lead to higher levels of microbes, particularly multidrug-resistant organisms (MDROs). Surgical wards, on the other hand, suffer from elevated microbial concentrations due to surgical site infections (SSIs), which are affected by the type of procedure performed and the condition of patient’s overall health. These discrepancies indicate how specialized medical activities influence the levels of microbes, which in turn affects indoor air quality (IAQ) and the risk of infection in hospital environments. 

Materials and Methods

Study Design and Setting

This study was conducted at a hospital located in Klang Valley, Malaysia, and included the Medical Ward (MW), Surgery Ward (SW), Orthopaedic Ward (OW) and Emergency Ward (EW). The selected wards accommodated high patient and healthcare worker densities, belonged to different medical disciplines and were ventilated either through natural airflow systems or centralized heating, ventilation, and air conditioning (HVAC) mechanisms.

Sampling Points

A total of eleven sampling sites (one for each ward) were selected in accordance with the total area specified by ICOP IAQ (2010) standards.²⁰ 

Measurement of Parameters

Physical Parameters

The physical parameters (measured by direct reading method) were as follows:

Temperature: Ambient air temperature was recorded.

Relative Humidity (Rh): Hygrometer was used to measure the moisture content in the air.

Air Movement: To evaluate ventilation effectiveness, air velocity in every ward was measured.

Chemical Parameters

The chemical parameters, including CO₂ (monitored by direct reading method) were assessed as follows:

Total Volatile Organic Compounds (TVOC) and Ozone (O₃): Assessed with an Aeroqual Series 500.

Carbon Monoxide (CO), Carbon Dioxide (CO₂), Particulate Matter (PM₁₀): Measured with a Tetra 3 Crowcon.

Formaldehyde (CH₂O): Measured with a Formaldemeter.

Both physical and chemical parameters were measured in the morning and evening. To enhance measurement reliability,  data were collected three times at each sampling point, with readings taken every five minutes over a 15-minute sampling period.

Fungal and Bacterial Analysis

The assessments of total fungal counts (TFC) and total bacterial counts (TBC) were carried out using the Quick Take 30 sampler. Duplicate Trypticase Soy Agar (TSA) media which served to culture fungal and bacterial present in the air samples in this exposure method. The colony of fungal and bacterial that determined in the plate were counted by the software integrated from  WIGGENS GmbH which is Galaxy 230 Colony Counter.

Statistical Analysis

All data were analysed using IBM SPSS Statistics version 26.0 software. Non-parametric tests were used, including the Mann–Whitney test, to look for differences based on ventilation type. A multiple linear regression analysis was conducted to identify the environmental predictors that significantly influenced microbial counts across hospital wards. This statistical approach is used to assess how ventilation type, especially natural ventilation, affects environmental parameters and airborne microbial levels within hospital wards.

Results

Indoor Air Quality and Airborne Pathogen Dynamics in Surgical, Medical, Orthopaedic, and Emergency Wards with Different Ventilation Modes

We investigated a total of eleven hospital wards, including surgical, medical, orthopaedic, and emergency department, to discover how different ventilation systems impact indoor air quality (IAQ). Six wards (WA, WB, WC, WD, WE, WF) used natural ventilation, while five (WR1, WR2, WR3, WH, WG) used mechanical air-conditioning systems. The measurements were carried out in the morning and evening, and all of parameter values were compared to the standard set by the ICOP IAQ (2010). Tables 1 and 2 show the mean value and standard deviations for physical (temperature, relative humidity, and air movement), chemical (carbon monoxide [CO], Ozone [O₃], formaldehyde [CH₂O] and total volatile organic compounds [TVOC] and particulate matter [PM₁₀]), including carbon dioxide [CO₂], and biological (TFC and TBC) parameters.

The physical parameters indicated significant variations in airflow pattern and thermal conditions between naturally and mechanically ventilated wards. Significant differences were found in temperature parameters within wards and the type of ventilation used. In wards with natural ventilation, the morning temperatures were between 30.2 ± 1.0°C to 32.3 ± 0.57°C, with a slight increase in the evening to 32.7 ± 0.86°C–33.1 ± 0.67°C, which is higher than the ICOP IAQ (2010) comfort range of 23–26°C. Ward WC had the highest mean temperature (33.1 ± 0.67°C), while Ward WD had the lowest temperature (30.2 ± 1.02°C). In contrast, mechanically ventilated wards remained cooler, with morning temperatures ranging from 22.1 ± 0.02°C to 24.8 ± 0.28°C, while evening value were from 21.9 ± 0.19°C to 25.2 ± 0.42°C, well within the acceptable comfort range. Although wards with air-conditioned environments offered superior thermal comfort, the higher temperatures in naturally ventilated wards allowed buoyancy-driven airflow, promoting vertical mixing and dilution of airborne contaminants. This was evidenced by the Mann–Whitney test showing a significant difference (Z = –6.462, p< 0.000), with naturally ventilated wards had higher mean ranks (44.50 vs 10.50). This indicates that natural ventilation made the air warmer but also improved convective flow (Table 3).

Relative humidity (Rh) in all wards remained generally within or close to the standard recommended range of 40–70%. Naturally ventilated wards exhibited a wider variability (59–73%) in comparison to mechanically ventilated wards (60–67%), indicating a greater interaction with outdoor climatic conditions. In the morning, Rh in naturally ventilated wards ranged from 61.6 ± 2.09% (WC) to 73.4 ± 2.40% (WE), while in mechanically ventilated wards, the Rh showed narrower values between 62.0 ± 0.52% (WR3) and 67.7 ± 0.19% (WH). Ward that recorded the highest Rh value in naturally ventilated ward was ward WE (73.4 ± 2.40%), while for mechanical ventilated wards, the highest Rh value was shown in a WG ward in the evening  (67.3 ± 0.33%). The Mann–Whitney test found no statistically significant difference between systems (Z = –0.471, p = 0.638), and Rh levels remaining relatively consistent within all wards (Table 3).

Air movement, as a key indicator of ventilation efficiency, was significantly higher in naturally ventilated wards (0.13–0.37 m/s) than in mechanically ventilated wards (0.05–0.15 m/s). Five out of six naturally ventilated wards were having airflow rates within the standard recommended range of 0.15–0.50 m/s. This indicates that the air exchange was sufficiently adequate to dilute and remove airborne particulates as well as microorganisms. This study found out that ward WD had the highest mean velocity (0.37 ± 0.09 m/s, PM), followed by Ward WB (0.31 ± 0.05 m/s, AM), indicating an effective cross-ventilation during active occupancy. Conversely, wards with mechanical ventilation had consistently low air velocities, with the lowest recorded in Ward WR1 (0.05 ± 0.01 m/s). This indicates that air mixing is limited within enclosed HVAC environments. The Mann–Whitney test revealed a very strong difference (Z = –6.071, p < 0.000), with naturally ventilated wards having a higher mean rank (43.90) than mechanically ventilated wards (11.95) (Table 3). This confirms that natural ventilation is better at diluting air and more effective at removing airborne contaminants than mechanical systems.

Chemical parameters further emphasised the performance advantage of natural ventilation, showing its superior ability to dilute indoor-generated contaminants relative to mechanically ventilated systems. Carbon dioxide (CO₂) concentrations indicated a clear contrasts between ventilation types. In naturally ventilated wards, the concentration CO₂ in the air in the morning ranged from 322 ± 19.69 ppm to 383 ± 44.95 ppm. This is far lower than those measured in mechanically ventilated wards, which recorded higher concentrations ranged from 476 ± 15.32 ppm to 918 ± 89.57 ppm. In the evening, naturally ventilated wards kept CO₂ levels low (295–341 ppm), while mechanically ventilated wards remained elevated (478–775 ppm). Although all wards complied with the standard of 1000 ppm, the noticeable elevations in mechanically ventilated spaces indicate inadequate dilution of exhaled CO_2, indicative of closed HVAC systems with limited outdoor-air exchange. The Mann–Whitney test showed a significant difference (Z = –6.461, p < 0.000), with naturally ventilated wards exhibiting a lower mean rank (24.50 vs 58.50). This result demonstrates that natural ventilation is more effective at air exchange and dilution capacity than mechanical systems, which tend to retain exhaled air (Table 3).

Carbon monoxide (CO) levels were negligible across all wards and continually well below the standard threshold of 10 ppm, demonstrating the absence of combustion-related contamination. Minor increases noticed in a few naturally ventilated wards (≤ 2.28 ppm) were probably caused by exhaust infiltration from cars on the nearby roads, rather than indoor combustion sources. Overall, these CO findings verify that both ventilation systems maintained safe exposure profiles. The Mann–Whitney test demonstrated that mean rank values were marginally higher in naturally ventilated wards (39.50 vs 22.50, Z = –3.785, p < 0.000) (Table 3), reflecting the occasional outdoor infiltration effects associated with external traffic emissions.

Ozone (O₃) levels fluctuated from 0.01 ppm to 1.91 ppm, and during the day in natural ventilated wards occasionally rose above the 0.05 ppm standard level. This was probably due to outdoor photochemical ozone intrusion. Mechanically ventilated wards, on the other hand, kept O₃ levels close to zero, since it filtered the air well and limited outdoor-air exchange, except for Ward WR1 (0.07 ± 0.00 ppm), went slightly above the guideline value. The Mann–Whitney test corroborated these results, demonstrating significantly higher mean ranks for naturally ventilated wards (43.52 vs 12.85, Z = –5.828, p < 0.000) (Table 3), and verifying greater ozone intrusion linked to direct ambient air exchange.

The biggest distinction between the two ventilation types was in the TVOC level. The average morning TVOC levels in naturally ventilated wards were recorded between 104.6 ± 88.9 ppm and 283.4 ± 288.2 ppm. In contrast, mechanically ventilated wards showed far higher levels, peaking at 385.4 ± 255.2 ppm (WR3) in the morning and reaching extreme evening levels of 545 ± 137.1 ppm (WH). These levels exceeded the standard  guideline of 3 ppm. Conversely, TVOC levels in naturally ventilated wards significantly decreased in the evening (24.8–82.9 ppm), indicating the effectiveness of open-air pathways in removing accumulated pollutants. The Mann–Whitney test revealed a significant difference (Z = –5.047, p < 0.000), with naturally ventilated wards having lower mean ranks (26.69 vs 53.25) (Table 3). This shows that mechanical ventilation without sufficient make-up air or proper filtration can raise the risk of chemical exposure, even while maintaining thermal comfort.

Formaldehyde (CH₂O) concentrations remained consistently low and were typically below the permissible limit of 0.1 ppm. However, two naturally ventilated wards were observed to have minor exceedances, namely wards WF = 0.16 ± 0.05 ppm and WB = 0.14 ± 0.01 ppm. The consistently low levels of CH₂O in all wards suggest that effective chemical dispersion and rapid degradation. This indicates that both ventilation systems maintained aldehyde levels within the standard requirements. The Mann–Whitney test indicated that naturally ventilated wards exhibited higher mean ranks (44.17 vs 11.30, Z = –6.287, p < 0.000) (Table 3), which may be attributed to occasional off-gassing and outdoor aldehyde infiltration.

The levels of PM₁₀ concentrations likewise stated below the standard  limit (≤ 0.15 mg/m³) in all wards. In naturally ventilated areas, they ranged from 0.02 to 0.11 mg/m³, and in mechanically ventilated areas they ranged from 0.01 to 0.07 mg/m³. The slightly higher concentrations in naturally ventilated wards were caused by minor resuspension of settled dust and infiltration of outdoor particles. Overall, the consistently low PM₁₀ concentrations in all wards verify that the air was effectively diluted and minimal resuspended of particle. These results indicates that both ventilation systems successfully maintained particulate levels within acceptable indoor air quality limits. The Mann–Whitney test showed that the mean ranks were significantly greater under natural ventilation (42.06 vs 16.35, Z = –4.886, p < 0.000) (Table 3). This suggests minor outdoor dust ingress, does not exceed safety thresholds.

The biological findings verified the trends observed in the chemical parameters, reflecting the combined influence of air movement, occupant activity, and ventilation efficiency on microbial dispersion. The concentration of airborne bacterial varied greatly between wards and types of ventilation. In naturally ventilated wards, morning bacterial loads were generally greater, ranging from 34.5 ± 3.24 CFU/m³ (WD) to 163.1 ± 1.73 CFU/m³ (WA). Mechanically ventilated wards also recorded higher morning bacterial counts, between 45.0 ± 3.96 CFU/m³ and 143.4 ± 1.00 CFU/m³, suggesting limited air dilution within recirculated systems. Despite these variations, all wards met the ICOP IAQ (2010) microbial threshold of ≤ 500 CFU/m³, indicating all wards were under acceptable bacterial air quality limit across both ventilation strategies. The Mann–Whitney test showed no statistically significant difference between systems (Z = –1.751, p = 0.080). However, naturally ventilated wards exhibited higher mean ranks (37.21 vs 28.00) (Table 3).

The concentrations pattern of fungal were also similar and remained well below the standard limit of 1000 CFU/m³. The overall levels ranged from 6.6 CFU/m³ to 96.2 CFU/m³. Natural ventilated wards WA had the highest concentration of fungi (96 CFU/m³, AM). Mechanically ventilated wards consistently exhibited low fungal counts (< 20 CFU/m³), while naturally ventilated wards demonstrated modest temporal variation, reflecting active air exchange and dynamic airflow that reduced spore stagnation. Although natural ventilation may permit limited ingress of outdoor spores, the overall amount of fungal load stayed low and clinically insignificant. The Mann–Whitney test showed statistically significant differences (Z = –4.428, p < 0.001), with naturally ventilated wards having a higher mean rank (41.34 vs 18.08) (Table 3). This confirms that natural air exchange processes are associated with greater fungal variability.

Table 1: IAQ parameters reading across hospital wards in the morning Click here to View Table

Table 2: IAQ parameters reading across hospital wards in the evening Click here to View Table

Table 3: The Mann – Whitney test comparing IAQ parameters concentration between natural and mechanical (air-conditioned) ventilated hospital wards.

*Significant at p < 0.01

Regression Analysis of Environmental Predictors on Microbial Counts

The Mann–Whitney test showed statistically significant differences for microbial counts between ventilation types (natural and air-conditioned) for TFC only. To discover the environmental predictors that significantly influenced the fungal counts across hospital wards, a multiple linear regression analysis was performed (Table 4). The final model kept five predictors – temperature, CO₂, CH₂O, PM₁₀, and room density. This was achieved using the backward elimination method (criterion: probability of F-to-remove ≥ 0.10). The regression model was statistically significant (R² = 0.162; F(5,62) = 2.397; p = 0.0047), indicating that these environmental factors accounted approximately 16.2% of the differences in fungal counts. Temperature exhibited a positive significant correlation with fungal counts (β = 2.823, p = 0.019), suggesting that high indoor temperature promote temporary increases in fungal spore concentration probably due to enhanced dispersion and metabolic activity at higher temperatures. In contrast, CH₂O showed a significant negative association (β = –154.249, p = 0.041), which means that aldehyde-based cleaning or disinfection solutions may prevent fungi from growing and propagating.

The other three predictors, namely CO₂ (β = 0.043, p = 0.161), PM₁₀ (β = 124.303, p = 0.230), and room density (β = 2.424, p = 0.073), were not significant, but they did exhibit positive coefficients, which means that particulate levels and higher occupancy may have impact on the prevalence of fungal. These results reveal that chemical composition, specifically CH₂O and temperature, have the biggest impact on shaping fungal behaviour; however, the effects of occupant load and particle concentration are less clear. Overall, the regression model underscores that environmental microclimate and chemical disinfectant activity are key determinants of fungal variability within hospital environments, reflecting the complex interplay between thermal conditions, pollutant dynamics, and microbial ecology.

Table 4: Multiple linear regression analysis of environmental predictors of fungal concentration in hospital wards

Dependent variable: Fungal count 

Discussion

This study presents compelling evidence that the type of ventilation significantly impacts IAQ and microbiological safety in hospital settings. Across eleven wards from surgical, medical, orthopaedic, and emergency departments, naturally ventilated wards consistently exhibited higher air dilution performance compared to mechanically ventilated wards. The Mann–Whitney tests revealed significant differences (p < 0.05) among nearly all of the IAQ parameters. Regression analysis identified that temperature and CH₂O were significant predictors of fungal variability (R² = 0.162, p = 0.0047). These findings collectively validate the hypothesis that natural ventilation is essential in mitigating airborne pathogens by improving airflow, pollutant dispersion, and microbial dilution efficiency. The investigation of physical parameters has further supported these statistical results by explaining detail the environmental factors that cause the discrepancies between the two forms of ventilation.

The physical parameters measured in this study align with the findings of Lv et al. (2022), who reported that naturally ventilated buildings have greater airflow variability than HVAC-driven systems.²¹ In this study, air velocities in naturally ventilated wards were significantly greater than those in mechanically ventilated wards, ranging from 0.13 to 0.37 m/s, compared to 0.05–0.15 m/s. Savanti et al. (2022) also found similar changes in airflow under natural ventilation. They observed variable air movement were between 0.03 m/s and 0.26 m/s at a public hospital.²²

The higher temperatures recorded in naturally ventilated wards (30.2–33.1°C) further support the presence of buoyancy-driven convection. This is in line with what Chew (2023) reported, namely that warmer indoor air enhances upward aerosol dispersion.²³ On the other hand, mechanically ventilated wards maintained stable thermal comfort (22–25°C), but this stability could lead to the accumulation of indoor air pollutants and consequently worsen the IAQ.²⁴ These physical dynamics not only define the movement and distribution of air within the wards, but also directly affect the concentration and behaviour of chemical contaminants, establishing a clear pathway through which ventilation type governs overall IAQ.

The chemical parameters identified in this study further validate the impact of ventilation type on pollutant dynamics. Naturally ventilated wards consistently demonstrated lower concentrations of CH₂O and TVOC, including CO_2, indicating more effective dilution and contaminant removal, alongside their significant statistical differences confirmed by Mann–Whitney tests (p < 0.05).

Carbon dioxide (CO₂) is a dependable tracer of ventilation efficiency and human occupancy. In this study, naturally ventilated wards exhibited CO₂ concentrations ranging from 295–383 ppm, significantly lower than the 476–918 ppm measured in mechanically ventilated wards. This suggests more effective air exchange and reduced rebreathing of exhaled air. Although all concentrations remained below the ICOP IAQ (2010) standard of 1000 ppm, the relative elevation under HVAC operation indicated insufficient outdoor-air supply compared to natural ventilation. This is a well-documented limitation of effectiveness recirculating mechanical systems. These findings are consistent with Roberts et al. (2022), who observed a significant reduction in CO₂ levels from 2000 ppm to 500 ppm upon opening windows at 6:00 a.m. in the Paediatrics ward of Tamale Hospital, Ghana following the earlier use of air conditioning.²⁵ Three high CO₂ peaks were also documented in the air-conditioned Emergency Department at Accra Hospital in Ghana, where windows stayed closed all day. Both observations reinforce that wards with natural ventilation maintain lower CO₂  levels than those with mechanical ventilation. This shows that naturally ventilated areas are better at diluting exhaled pollutants and improving IAQ.²⁵ In Lahore, Pakistan, six hospitals (four public and two private) with mechanical ventilation also experienced elevated CO₂ levels, ranging from 712 ppm to 1093 ppm.²⁶ Comparable conditions were observed in Malaysia. Mechanically ventilated healthcare clinics, Medical Clinic 1 and Medical Clinic 2 at University Hospital, Klang Valley, both had CO₂ levels of 1668 ± 94.60 ppm and 1484 ± 101.83 ppm, which were  both greater than the acceptable ICOP IAQ level.²⁷ The consistent increase in CO₂ levels in mechanically ventilated areas suggests limited outdoor air exchange, which could also lead to the buildup of other indoor-generated pollutants such as CH₂O and TVOC. Hence, the subsequent analysis of these chemical parameters further elucidates how ventilation mode governs the chemical composition and overall quality of hospital indoor air.

The most noticeable difference between the two forms of ventilation was in terms of TVOC. Mechanically ventilated wards had very extreme high evening concentrations of up to 545 ppm, far exceeding the standard limit of 3 ppm. On the other hand, naturally ventilated wards showed major evening reductions (24.8–82.9 ppm). This trend suggests that natural ventilation facilitates effective dilution of pollutants that have built up over the day, which is in line with studies that claim open airflow enhances contaminants dispersion in tropical hospital settings.²⁸ Supporting this, Riveron et al. (2021) found that the TVOC levels were significantly lower in the summer (634.2 ± 682.9 µg/m³) than in the winter (1,089.0 ± 1,582.8 µg/m³) at Glenfield General Hospital (GGH) and Leicester Royal Infirmary (LRI), United Kingdom. They attributed this seasonal decline to increased window opening and natural airflow, which complemented the mechanical ventilation system in use.²⁹ Moldovan et al. (2024) have also highlighted the urgent need for effective and flexible ventilation techniques in hospitals to minimize chemical pollution to a minimum.³⁰ These results collectively, underscore the critical importance of natural ventilation in mitigating VOC accumulation, especially in tropical climates where solar-driven buoyancy and cross-ventilation facilitate pollutant removal and enhance IAQ stability.

Formaldehyde (CH₂O) levels remained low and within the standard limit of 0.1 ppm, which meant that both ventilation systems were able to regulate the chemicals well. Two naturally ventilated wards WF (0.16 ± 0.05 ppm) and WB (0.14 ± 0.01 ppm) showed minor exceedances – possible due to transient off-gassing from disinfectants or infiltration from nearby traffic and chemical sources. The Mann–Whitney test indicated a significant difference (Z = –6.287, p < 0.001), with naturally ventilated wards exhibiting slightly higher mean ranks (44.17), demonstrating greater variability due to open-air exchange and external environmental influence.

Despite these minor fluctuations, CH₂O levels in both ventilation systems were substantially lower than those reported in other hospital settings; for example, Rao et al. (2023) recorded mean CH₂O concentrations as high as 2.4 ppm in operating rooms in Puducherry, India – particularly 1.2 ppm near anaesthesia workstations and 1.0 ppm adjacent to surgical teams during electrocautery use – values that far exceed recommended safety limits, underscoring the risks of pollutant buildup in sealed HVAC environments.³¹ The high levels in that study were due to the fact that confined nature of mechanically ventilated operating rooms, emphasizing the need for improved exhaust and dilution design. The current findings, however, show that both natural and mechanical ventilation systems in Malaysian hospitals successfully kept aldehyde levels well below the safety limits set by the government. This demonstrates efficient pollutant dispersion and degradation under tropical climatic conditions.

Regression analysis elucidated the relationship between CH₂O and TFC. A significant negative association (β = –154.249, p = 0.041) demonstrated that elevated CH₂O concentrations were associated with reduced fungal loads, indicating that aldehyde-based cleaning and disinfection agents exert inhibitory effects on fungal viability and sporulation. This relationship illustrates how ventilation type modulates chemical-biological interactions. Naturally ventilated wards, though more susceptible to short-term aldehyde fluctuations, benefit from rapid pollutant dilution, which prevents sustained accumulation.  Mechanically ventilated areas better maintain chemical stability, but risk longer pollutant residence times if airflow is insufficient. This equilibrium between chemical suppression and dilution efficiency shows how ventilation strategy and disinfection collaborates together in maintaining indoor microbial safety. Khalil et al. (2022) documented significant decreases in airborne mold concentrations subsequent to formaldehyde-based disinfection, demonstrating efficacy at exposure durations of 15 minutes, 6 hours, and 24 hours.³² In summary, these results indicate that there is a dynamic balance between controlling pollutant and microbial inhibition. Natural ventilation ensures dispersion, while chemical agents make the air more sterile. Together, these factors maintain safe IAQ conditions in hospital surroundings.

Particulate matter (PM₁₀) is a key indicator of IAQ and is known to transport biological and chemical pollutants in healthcare environments. In this study, PM₁₀  levels in natural or mechanical ventilation wards stayed below the standard limit of 0.15 mg/m³. This indicate that all the wards were compliant with regulatory standards. However, there were large discrepancies between the two modes of ventilation. The Mann–Whitney test indicated a significant difference (Z = –4.886, p < 0.001) between naturally and mechanically ventilated wards, with a higher mean ranks for natural ventilation (42.06) compared to mechanical ventilation (16.35). This finding indicates that although natural ventilation enhances air exchange, it may also introduce transient outdoor particulates through open windows and doors, especially in wards located near vehicular zones or high foot-traffic areas. These results are consistent with findings from Chamseddine et al. (2019), who reported that PM₁₀ concentrations in mechanically ventilated hospitals were lower (range: 24-55 mg/m3) compared to naturally ventilated hospitals (range: 28-94 mg/m3).³³ The current study demonstrated that despite the higher mean rank observed under natural ventilation, PM₁₀ concentrations remained below the permissible threshold, indicating that particles from the outdoors were rapidly diluted through enhanced air movement and buoyancy-driven airflow. Regression analysis made it even clearer that PM₁₀ is not an effective predictor of airborne fungal levels. The model found a positive but not statistically significant association (β = 124.303, p = 0.230), which indicates that higher PM₁₀ levels tended to be correlated with increased fungal levels. However, this association was weaker compared to the relationship with temperature (β = 2.823, p = 0.019) and CH₂O (β = –154.249, p = 0.041), which were significant predictors of fungal variability (R² = 0.162, F(5,62) = 2.397, p = 0.0047). The minor influence of PM₁₀  indicates that particles may act as temporary carriers for fungal spores under dynamic air conditions, but when effective ventilation and continuous air movement are maintained, their contribution to the overall microbial load is minimal. Overall, these data suggest that natural ventilation, despite slightly greater particulate variation, still provides superior dilution capacity and pollutant dispersion efficiency. This supports its use as an effective and long-term strategy to keep the indoor air healthy in hospital environments.

Microbial findings in this study were below the standard limit and parallel those of Kotgire et al. (2020), who observed that bacterial and fungal concentrations in naturally ventilated wards fluctuate throughout the day with higher concentrations typically recorded in the morning compared to the evening.³⁴ In this investigation, the morning bacterial counts (34.5–163.1 CFU/m³) were higher in naturally ventilated areas because of medical activities and occupancy. This is in line with Ye et al.’s (2025) findings, which showed similar transient peaks after morning operations and staff activity.³⁵ By evening, microbial counts declined significantly, reflecting the self-purging capacity of open-air ventilation.²⁸ Bioaerosol count in this study ranged between 19.9–163.1 CFU/m³ (bacteria) and 6.6-96.2 CFU/m³ (fungal). The magnitude of bioaerosol concentrations in this study were markedly lower compared to the findings by Bozic and Ilić (2019), where fungal counts ranging from 20-1125 CFU/m³, while bacterial levels varied between 30 – 6295 CFU/m³.³⁶ The results of this study also revealed that areas without HVAC systems moderately higher fungal and bacterial counts than those with mechanical ventilation. This trend aligns closely with the findings of Bozic and Ilić (2019), which demonstrated that wards with air handling units experienced a decline in fungal counts, whereas natural ventilated spaces exhibited increased airborne fungal concentrations.³⁶

HVAC systems are effective in managing thermal comfort and filter contaminants,  as also claimed by Shajahan et al. (2019), who found out that different HVAC configurations can reduce or eliminate pathogenic microorganisms.³⁷ the complexity of indoor air pollutants, particularly biological agents, continues to challenge researchers. Furthermore, their diverse natures, behaviours, and release mechanisms make them difficult to predict and control.³⁸ This complexity is compounded by the recent emergence of new and identified respiratory diseases³⁹, which underscores the continuing vulnerability of the indoor environment. Steiner et al. (2020) has also demonstrated that the human respiratory system is highly complex and readily absorbs airborne droplets and particulates, thereby amplifying the health risks posed by enclosed spaces.⁴⁰ More concerningly, Argyropoulos et al. (2023) claimed that as of 2023, global efforts are still ongoing to prevent and regulate the emission of chemical pollutants and biological agents, including pathogenic microorganisms.³⁸ This indicates that stakeholders must improve ventilation strategies. From the results of this study, we can conclude that natural ventilation offers a more dynamic, energy-efficient, and biologically self-cleansing system, suited for tropical climates. However, hybrid ventilation combining natural airflow with mechanical filtration may be more efficient, which can improve IAQ, reduce airborne infection risks, and protect the health of both patients and healthcare workers.

The regression models statistical significance (p=0.0047) indicates that environmental conditions established by different ventilation modes have a big effect on differences in fungal variability. Temperature was identified as a significant predictor (β=2.823, p=0.019), suggesting that elevated temperature correlate with higher instantaneous fungal concentrations. But this relationship is probably due to the increased air exchange and spore coming in from outdoor rather than enhanced indoor fungal proliferation. Wards with naturally ventilation have higher temperatures and stronger buoyancy-driven airflow,²³ which allows greater dispersion and dilution of airborne microorganisms.⁴¹ This leads to a reduction in long-term fungal persistence. Thus, while temperature positively correlates with momentary fungal counts, it simultaneously supports a more dynamic and self-cleansing ventilation environment that suppresses microbial accumulation. Vourinen et al. (2020) also pointed out that changes in temperature can influence the activation or deactivation of bioaerosols.⁴² However, this interpretation contrasts with the findings of Alrayess et al. (2022), who reported that natural ventilation and fan-coil units can, under specific climate or conditions, aggravate indoor fungal concentrations.⁴³ On the other hand, mechanically ventilated wards operate under cooler and more stable thermal conditions that keep outdoor-air from coming in, but allow air recirculation. Unfortunately, if systems are poorly maintained, potential microbial buildup is likely to occur.³⁶ Therefore, even if higher temperatures are connected to short-term increases in the number of fungal, this indirectly enhances the efficiency of natural ventilation in overall dilution to reduce long-term microbial persistence. This perspective emphasises that temperature, as a physical agent of natural ventilation, improves dilution efficiency rather than biological amplification.

Conclusion

This study presents strong empirical evidence indicating that the type of ventilation significantly affects IAQ and microbial safety in hospital settings. Wards with natural ventilation consistently showed greater air dilution, pollutant dispersion, and microbial control than wards with mechanical ventilation. Statistical tests indicated significant differences between IAQ parameters, revealing temperature and formaldehyde as the main predictors of fungal variability, and close interactions between thermal conditions, chemical composition, and biological responses.

Acknowledgment

We thank our colleagues at the National Institute of Occupational Safety and Health (NIOSH), Bandar Baru Bangi, Malaysia, for their help with this research, including their expertise, technical support, and equipment. We also acknowledge the hospital in Klang Valley that took part in asissting with data collecting and supporting the study.

Funding Sources

This research was supported by Universiti Kebangsaan Malaysia, Malaysia (GRANT: TAP-K020627 and TAP-K006423)

Conflict of Interest

The author(s) do not have any conflict of interest.

Data Availability Statement

This statement does not apply to this article. 

Ethics Approval

The study was conducted following ethical guidelines, securing approval from the National University of Malaysia (Reference No: JEP-2020-131). This included adherence to ethical standards in research involving human environments and the collection of environmental data.

Informed Consent Statement

This study did not involve human participants, and therefore, informed consent was not required.

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