Clearing the air in contaminated spaces

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Preventing the spread of a respiratory pathogen in a veterinary hospital has always been a challenge. When a patient exhales or barks, they expel a respiratory plume that contains a mix of gases, aerosols, and droplets. Pathogens are carried within these aerosols and droplets and can rapidly spread throughout the hospital.¹

The patient is often in multiple areas of the hospital from the time they enter the lobby until they are discharged. To isolate the patient, many hospitals will leave the patient in the examination room as much as possible. Once the patient has been discharged or hospitalized, the exam room is left closed for 24 hours and is cleaned multiple times during this period. This protocol leaves the hospital down an exam room and may contribute to the spread of respiratory pathogens.

There are multiple ways infectious diseases spread, including via vectors, fomites, direct contact, and fecal-oral and aerosol transmission. Of these, aerosol transmission is perhaps the most challenging to control in the veterinary setting. Prevention of disease transmission via respiratory droplets or aerosols is difficult, if not impossible, to achieve with isolation alone. In veterinary medicine, there are multiple papers available that discuss airborne transmission of pathogens; however, most focus on livestock and poultry.^2-4

Controlling respiratory pathogens

Veterinary research that investigates the control of small animal pathogens typically includes a review of transmission, replication, clinical signs, treatment, facility design, isolation protocols, and vaccination.^5-7 Multiple factors affect the spread of a respiratory pathogen, and the advent of COVID-19 has led to the publication of numerous research papers that investigate these factors.^1,8-11 It is necessary to extrapolate and apply information obtained in human research for our purposes because of a deficiency of information specific to veterinary medicine.

The deposition of droplets and aerosols is dependent on their size. Expelled respiratory fluid greater than 5 μm is considered a droplet and will fall to a surface within several minutes. Expelled respiratory fluid less than 5 μm is an aerosol and will remain suspended in the air for several hours.¹ Asadi et al found the average diameter of particles expelled during breathing to be 1 μm, falling into the category of an aerosol.¹² Based on similar respiratory mechanics in humans and small animals, it can be extrapolated that the average particle size expelled from a small animal patient during breathing is likely comparable.

The probability of the transmission of a disease via respiratory secretions can be calculated mathematically using the Contagion Airborne Transmission inequality formula.¹³ This complex equation is used in human research and allows multiple factors to be considered when calculating the likelihood of transmission of a pathogen. Included in this equation are several environmental factors, including temperature, relative humidity, air movement, air filtration, and the deposition of respiratory secretions. Viruses tend to survive longer and spread more easily at lower temperatures because of a delay in decay.^12-14 Additionally, if the exhaled aerosol plume is warmer than the temperature of the room, it will rise and be caught in air currents that then move throughout the hospital.¹

Environmental factors

Relative humidity has a profound and complex effect on the spread of pathogens, and it is important to mention that relative humidity contributes to the speed at which a particle will dehydrate, which is a critical factor. Larger droplets will land on a surface before they dehydrate, depositing pathogens on the surface. Smaller aerosols dehydrate while still suspended in the air, leaving the pathogens contained inside airborne, along with the lipids, albumins, and other fluids, thus increasing the risk of transmission.⁸

Multiple studies have shown that relative humidity affects the spread of respiratory aerosols and droplets, in that a lower relative humidity (< 50%) will decrease the time required for a droplet to evaporate and increase the speed at which it dehydrates.^8,14-18 Respiratory aerosols as small as 1 μm will evaporate in less than 1 second,¹² which allows them to remain suspended in the air for a longer period, increasing the likelihood of infection.¹⁷ At a higher relative humidity, the aerosol or droplet may absorb water and increase in size, causing it to fall to a surface more quickly.¹⁴ Additionally, relative humidity appears to affect the survival of viruses and bacteria differently, making it difficult to determine the ideal relative humidity to help prevent or decrease transmission.¹⁶

Air currents also play a role in determining where a respiratory aerosol will travel. When a patient breathes, coughs, or vocalizes, a humidified plume of virus-laden respiratory aerosols and droplets are expelled. These plumes leave the upper respiratory system at varying speeds and trajectories. During normal respiration, plumes are released at a low speed, whereas vocalizing or coughing releases the plume at higher speeds.¹² The initial speed at which the plume leaves the respiratory tract will determine how far the aerosols travel, in that those expelled with a higher speed will be carried farther.¹ One study from human medicine showed that as the volume of speech increases, the volume of the air plume expelled increases consequently, increasing the number of viral particles expelled. It is expected that a barking dog would release a larger number of viral particles compared with a dog breathing normally.¹²

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These infectious aerosols are airborne, and they readily move with air currents. Every time the heating, ventilation, and air conditioning (HVAC) system turns on or the door to an exam room is opened or closed, an air current is created, carrying pathogens throughout the hospital. Although it is not practical to eliminate air currents, an area of focus can be decreasing the number of pathogens in the air.

One study that looked at the transmission of airborne pathogens on public transportation found that the installation of adequate filters in the HVAC system resulted in a 93.95% reduction in the aerosol particle count compared with the count obtained before filters were installed.11 Although some HVAC systems are easily updated with new filters, installing new filters throughout a building may not be practical. One alternative is the placement of an air purifier in the exam room.

How an air purifier can help

A 2020 study observed a patient with a respiratory rate of 12 breaths per minute and a tidal volume of 1 L emitting 22,500 virions in 15 minutes. The use of a local air purifier decreased the virions in the air to 9000 within that 15-minute period, showing that the air purifier removed more than 50% of the airborne virions.¹⁰

However, choosing an air purifier can be daunting. Ideally, the air purifier would include a high-efficiency particulate air (HEPA) filter and prefilters (class F7 and/or F9), which will capture larger particles (> 10 μm) and extend the life of the HEPA filter. The Clean Air Delivery Rate (CADR) is the amount of cubic ft of air the machine will filter in 1 minute and is based on how much air flows through the filter and the efficacy of that filter. Home air purifiers will have a CADR for smoke (0.09-1 μm), dust (0.5-3 μm), and pollen (5-11 μm). Some researchers have recommended a purifier with a CADR of 1000 or above for the classroom setting.¹⁹ It is also recommended that the CADR be 2/3 used in that exam room can be lower. The HEPA filters should be changed every 4 to 8 weeks, and the prefilters will require weekly visual inspection and should be cleaned or replaced as needed.

Takeaways

The larger droplets that hit a surface before they dehydrate will leave their pathogens on the surface on which they fall. Most pathogens we see in veterinary medicine are susceptible to multiple disinfectants, and surfaces should be cleaned as recommended by the manufacturer of the disinfectant used at your hospital.

At the author’s hospital, air purifiers were selected based on room size needs, and the air purifier is turned on when a suspect infectious patient enters the exam room. When the patient leaves the room, the door remains closed with the air purifier running for 30 minutes. At the 30-minute mark, the air purifier is turned off, surfaces are cleaned, and the room is opened to the next patient.

At the time this paper was written, in fall 2023, an unknown respiratory outbreak was present. The hospitalized infectious patients were housed together in 1 ward and strict isolation protocols were followed. Based on the size of the room and the number of patients housed, 2 air purifiers were placed in the ward and were left running 24 hours a day. A formal review of cases was not pursued; however, no patients coming into the hospital during this time developed clinical signs associated with the respiratory outbreak.

References

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