Who Knew? UO study finds mid-range humidity removes viral particles indoors

A study at the University of Oregon looked at strategies to reduce viral particles in the air indoors. The research was conducted in a small, airtight, modular building on campus and involved 11 students diagnosed with COVID-19.

University or Oregon, Alen Mahic

For the study, infected students entered the building one at a time and were asked to talk loudly, cough and exercise on a treadmill. Researchers measured viral particles in the air, on surfaces and from each student’s nose and mouth.

They were measuring what variables might reduce viral particles indoors. Ventilation and air filtration worked. Lead researcher Kevin Van Den Wymelenberg said the exciting result came from humidity.

“Who knew that humidity mattered in COVID transmission risk?—he asked. “Increased water content in air means those viral particles get caught up in larger globules and fall out of the air more quickly so that you’re not likely to breathe them.”

The study found mid-range humidity, between 40 and 60 percent, is optimal for removing viral particles indoors.

Van Den Wymelenberg says there is evidence that mid-humidity range reduces the infectivity period of SARS-Cov-2 –which basically means rendering the virus less capable of disease transmission. He adds that humid air is helpful to the human immune system, keeping mucus membranes healthy.

Humidification and ventilation go hand in hand in controlling viral particles in the air, but they could counteract each other too. Van Der Wymelenberg says , “if the air is too humid, it increases the risk of mold and very dry air allows dry particles to float longer.”

The UO researchers had hypothesized that humidity is a factor in viral transmission risk --but this study is one of the first to observe the phenomenon in a “real-world setting.”

Credit University of Oregon, Alen Mahic

Kevin Van Den Wymelenberg is a professor of architecture and director of the UO’s Institute for Health and the Built Environment.

The study, published in the journal Clinical Infectious Diseases, was conducted by Van Den Wymelenberg, with UO staff and graduate students. The lead author was doctoral student in architecture, Hooman Parhizkar.

Science Source

For more, here’s the abstract:

Title: Quantifying human and environmental viral load relationships amidst mitigation strategies in a controlled chamber with participants having COVID-19

In this study, we recruited a cohort of 11 participants diagnosed with COVID-19 to individually occupy an environmentally controlled chamber for a period of three days each. Below is a summary of key questions and reported findings. Below that is the abstact.

• What is the relationship between human viral load and environmental contamination?
  
  • We report a significant coefficient for all nasal and aerosol samples in routine trials whereby an increase in nasal viral load equivalent to -1 C_Tis associated with an increase in room aerosol viral load of -0.36216 C_T(Supplemental figure 1). Furthermore, we report quantitative correlations between human viral load and high touched surfaces, as well as settling plates (Figure 3).
• Is there a difference in aerosol viral load at different distances from the infected participant, and is this affected by room air movement?
  
  • We report that an increase in viral load equivalent to -1 C_Tin human nasal samples is significantly associated with an increased near field viral load of -0.32639 C_Tand an increased far field viral load of -0.4014 C_Tamong routine trials (Figure 2a).
  • The difference of means between the aerosol C_Tvalue of near field and far field aerosol samples in routine trials was 1.0583 C_T, whereas far field samples represent lower viral load, however the paired t-test differentiating near field and far field samples was not significant (P = 0.05955). Interestingly, CO₂concentration and the number of fine particles show statistically significant differences between near field and far field (Figure 2).
• How much ventilation do we need to significantly reduce aerosol viral load, and does this differ by distance from the infected individual?
  
  • We observed that the aggregate of ventilation and filtration trials significantly reduced room aerosol viral load and that of select surfaces, when compared to control trials with ~0 ACH.
  • When examining total room aerosol viral load (near field and far field together), we report that trials with less than ~4.5 ACH (including ~0 ACH trials) were associated with statistically higher viral load, by nearly an order of magnitude, than trials with greater than ~9 ACH (mean difference of -3.2 C_T).We noticed that CO₂has been frequently discussed among expert communities as an indicator of appropriate ventilation. Therefore, we provide the first real world correlation between aerosol viral load and CO₂concentration that is affected by outdoor air exchange rates (figure 5b), where an increase in ~128 PPM of CO₂concentration generated by an individual with COVID-19 corresponds with an increase in aerosol viral load equivalent to -1 C_T, thus, approximately a doubling of the viral load.
• How much in-room filtration is needed to significantly reduce aerosol viral load, and does this differ by distance from the infected individual?
  
  • We report that HEPA filtration trials (with ~1000 m³/hr) had significantly lower room aerosol viral load, by nearly an order of magnitude, when compared with control trials without filtration (mean difference of 3.240741 C_T, Figure 5d).
• Does the level of indoor relative humidity significantly alter the aerosol viral load?
  
  • We report that increasing relative humidity by ~11.85% is significantly associated with approximately an 50% decrease in aerosol viral load (Figure 6a).

Abstract:Several studies indicate that COVID-19 is primarily transmitted within indoor spaces. Therefore, environmental characterization of SARS-CoV-2 viral load with respect to human activity, building parameters, and environmental mitigation strategies is critical to combat disease transmission. We recruited 11 participants diagnosed with COVID-19 to individually occupy a controlled chamber and conduct specified physical activities under a range of environmental conditions; we collected human and environmental samples over a period of three days for each participant. Here we show that increased viral load, measured by lower RNA cycle threshold (C_T) values, in nasal samples is associated with higher viral loads in environmental aerosols and surfaces captured in both the near field (1.2 m) and far field (3.5 m). At ambient conditions with ~0 Air Changes per Hour (ACH), near field measurements showed a higher particulate matter abundance and carbon dioxide (CO₂) concentration as compared to far field measurements. We also found that aerosol viral load in far field is correlated with the number of particulates within the range of 1 µm -2.5 µm. Furthermore, increased ventilation and filtration are associated with lower environmental viral loads, and higher relative humidity is associated with lower aerosol viral loads and higher surface viral loads, consistent with an increased rate of particle deposition. Data from near field aerosol trials with high expiratory activities suggest that respiratory particles of smaller sizes (0.3 µm -1 µm) best characterize the variance of near field aerosol viral load. Moreover, our findings indicate that building operation practices such as ventilation, filtration, and humidification substantially reduce the environmental aerosol viral load, and therefore inhalation dose, and should be prioritized to improve building health and safety.