From Droplets to “Through the Air”: Why Ventilation and Respirators Matter More Than Ever in Infection Prevention

One outcome of the SARS-CoV-2 pandemic is a renewed focus on preventing infections from pathogens that spread through the air. A new paradigm is emerging, with the abandonment of the pathogen-specific “droplet” guidance precautions in favor of one based on a continuum of particle sizes and the physical factors that project these particles into the air. To better describe this transmission, the World Health Organization has encouraged the use of the new terminology, “through the air.”

Particles of 100 μm or less can become aerosolized. Smaller particles remain in the air longer and may contain a higher concentration of an infectious agent. Viruses (such as SARS-CoV-2) have diameters of approximately 0.1 µm, whereas bacteria (such as methicillin-resistant Staphylococcus aureus) are larger, with diameters of approximately 1 µm. However, in a clinical setting, these pathogens most often reside in liquid droplets of varying sizes.

But to become airborne, an infectious agent needs to be subjected to an aerosolizing event, such as “breathing, talking, singing, spitting, coughing, or sneezing.” An aerosol-generating procedure (AGP), which can aerosolize almost any infectious agent, is not required. Activities such as flushing the toilet can also generate aerosols, necessitating high airflow in bathroom facilities.

Once these particles are airborne and transmitted, individuals can be exposed to the infectious agent. However, whether you become infected depends on the dosage of the particles you are exposed to. This dosage, or ID50 (infectious dose 50; number of microbial cells required to cause infection in 50% of a tested population), varies with different pathogens. Exposure dosage is also related to the severity of the contracted respiratory tract infection. Thus, even if an infectious agent cannot be eliminated from the environment, decreasing its concentration and, therefore, a patient’s exposure dosage may be beneficial. For example, masking during SARS-CoV-2 outbreaks was associated with a higher incidence of asymptomatic disease.

The infectious capacity of pathogens varies widely. In the US, there are currently 3 agents of utmost concern: seasonal flu, SARS-CoV-2, and measles. All have varying propensities to cause infections, and all can spread through the air (as well as on surfaces).

Because health care facilities are dealing with all 3 infectious agents simultaneously, their intervention strategies must address the agent with the greatest potential to spread. The basic reproduction number (R0; without mitigation) for measles is as high as 18 (infected individuals will, on average, spread the disease to 18 other people); for SARS-CoV-2, it is approximately 5 to 6; and the seasonal influenza R0 averages 1.3.

Optimal ventilation can significantly reduce the spread of seasonal flu. Jianyu Lai and colleagues recently reported data from a study regarding seasonal flu transmission. In one study arm, 1 infected participant exposed 8 recipient participants to influenza, and in another, 4 infected participants exposed 3 recipient participants to the virus. Recipients were quarantined in a hotel and were subjected to multiple exposure events lasting between 2 to 4 hours over a 2-week period. No transmission was observed to any of the participants. Room ventilation rates were low, between 0.25 to 0.5 air changes per hour (ACH). But there was rapid mixing of the air, which was actively circulated, and the patients coughed infrequently, limiting the viral dose to which participants were exposed. In addition, the recipient participants were middle-aged (mean age, 36) and may have had cross-immunity from decades of previous influenza exposure.

Thus, stopping the airborne transmission of seasonal influenza (with an R0 of 1.3) should be easier than stopping the transmission of SARS-CoV-2 or measles. The latter 2 pathogens are highly infectious and primarily spread through the air.

Health care facilities need to implement strategies to increase indoor ACH to the highest level possible. An air change is the volume of air in a room replaced with clean air. Unfortunately, these are not “complete” because the air circulation in a room is uneven. Within a single air change, some of the room air is cleaned or replaced 2 or 3 times, whereas other room air is not at all. It requires 4.6 air changes to clear 99% of a pathogen, and 6.9 air changes to clear 99.9% of pathogens. (See CDC 2003 Airborne Contaminant Removal Tables.) Even at a circulation rate of 15 ACH, a health care worker may be exposed to a dangerous pathogen for up to 28 minutes.

Unfortunately, measles is starting to achieve sustained transmission in the US. Although cases are still relatively infrequent, their severity leads patients to frequent health care facilities. Up to 90% of people who are not immune and are in the same room (nearby) as an infected individual will become infected.

Thus, to prevent the spread of pathogens that are transmitted through the air, one should wear a respirator (N95 mask) rather than a surgical mask.

ACH can be achieved by using HEPA (high-efficiency particulate air) air purifiers, outside air, or air treated with germicidal UV-C lights. Although all these interventions will reduce bacterial pathogens, using unfiltered outside air can allow industrial pollution to enter a room. However, CO2 levels are not reduced by HEPA filtration or germicidal UV-C, but they can be reduced by circulating outdoor air. There is also scientific agreement that carbon dioxide (CO2) should be kept below 800 ppm and as close to outdoor levels as possible. High levels of CO2 can decrease cognitive function. Airborne particles from wildfire smoke and industrial pollution can be removed by HEPA filtration, but not by germicidal UV-C lighting.

Thus, the exact type of heating, ventilation, and air conditioning (HVAC) system a facility uses is often determined by a facility’s location.

In health care facilities, the recommended minimum ACH has ranged from 6 to 15, depending on the type of service provided in the ventilated room. Procedural rooms require a minimum of 15 ACH, as emergency department waiting areas require a minimum of 12 ACH. However, with the reemergence of measles, these recommendations would be too low to prevent transmission. The American Society of Heating, Refrigerating, and Air-Conditioning Engineers (ASHRAE) has also published guidelines for the control of infectious aerosols, ASHRAE Standard 241, Control of Infectious Aerosols, which can be purchased from their website.

The concept of airborne pathogens as the “cost of doing business” was introduced by Ralph Abraham, MD, the principal deputy director of the CDC. However, Abraham erroneously stated that international travelers were bringing measles into the US. In actuality, the US is on the verge of losing its measles elimination status and may well be one of the countries contaminating the world.

With the emergence of SARS-CoV-2 and the sustained transmission of measles, the true cost of doing business will be the costly upgrading of HVAC systems, which will need to perform far beyond the 2003 CDC guidelines, as we now face the most infectious diseases known to humans.