As part of an infection prevention protocol, it’s vital that veterinary hospitals implement strategies that improve the safety of indoor air.

Clean air is a basic requirement of life and the quality of indoor air, where people and pets, spend a large part of their life is an essential determinant of health and well-being.¹ A debate is ongoing as to whether indoor air should be treated the same way as drinking water, from the decontamination point of view.² In any given setting, one may choose not to drink the water or eat the food that is available but generally has little choice in breathing the same air as everyone else. This makes air an environmental equalizer, conferring on it the unique potential to disperse evenly whatever it may contain, and infectious agents entering indoor air can mix rapidly with no perceptible color or smell.³

This highlights the importance for improving the indoor air quality and reducing the exposure to pollutants and pathogens.⁴ Indoor air quality has come to the forefront during the pandemic caused by an airborne pathogen and people adapting to a “new normal”. Veterinary companion animal medicine shifted to curb-side service in order to improve their team’s safety and maintain essential veterinary services.

The goal of creating a healthy indoor environment is nebulous, because individuals respond differently to different exposures and there is a lack of clear consensus as to what constitutes a healthy indoor environment and what measurable metric can be used to assess indoor air.⁵ Ideally multiple strategies, such as ventilation, filtration and decontamination, will need to be combined in a layered approach with other routine infections prevention measures with the end goal of making indoor spaces safer. Additionally, each building presents a unique and time-varying challenge and opportunity to improve the health of its occupants. 

AIRBORNE PATHOGENS

Buildings have been associated with infectious diseases outbreaks in humans and animals, such as COVID-19 or influenza, and the time people or animals remain indoors in close contact is a main contributor for contagion.⁶ Sources for indoor microbes are: aerosols from people and animals, pathogens aerosolization from biofilm and resuspension of dust.^3,6,7 In human hospitals, pathogens such as Acinetobacter baumannii, noroviruses, and Clostridium difficile responsible for health care associated infections can spread via aerosol.^7,8 Furthermore, airborne pathogens can settle on surfaces, which in turn become secondary vehicles for transmission. ⁸ Canine and feline viral respiratory disease, spread via aerosols, being a prevalent problem in shelters, boarding facilities and veterinary hospitals.⁹ 

Indoor air is a complex media that contains particulate and gaseous components, and it is a mixture of outdoor air and recycled indoor air.⁵ Large respirable droplets (>5 μm) rapidly settle out of the air, whereas virus-laden small droplets (<5 μm), referred to as “droplet nuclei” are suspended in the air for long periods of time and propagate depending on air-flow.¹⁰ In poorly ventilated and crowded indoor spaces, infectious virus within the “droplet-nuclei” can infect susceptible hosts.¹⁰ Hospitals are dynamic environments where bio-aerosols composition is diverse and generated from multiple sources such as patients, staff, outdoor air, surfaces, drains and equipment. Also season, temperature and humidity, ventilation system, number of people and animals, frequency of doors opening or movement of people influences the microbe concentration in the air.⁷ A major challenge in prevention and controlling the airborne spread of infection is the presence of multiple and mobile sources of pathogens at a given location and time.³ Infected or colonized people or pets may contaminate air in their immediate vicinity, exposing susceptible hosts without the air having reached any available means of pathogen decontamination. Therefore, when there is very close contact between people and or animals, it would be virtually impossible to prevent exposure to an airborne pathogen.²

INDOOR AIR MITIGATION STRATEGIES

Controlling the concentration of indoor respiratory aerosols to reduce airborne transmission of infectious agents is paramount to keep occupants safe. ⁶ This can be achieved via source control measures such as: face mask or distancing and engineering controls such as: ventilation, filtration or air decontamination (photocatalytic oxidation).⁶

Ventilation can reduce the concentration of pathogens in the air, decreasing the probability for pathogens to be inhaled, contact mucus membranes or fall out of the air to accumulate onto surfaces.¹¹ Ventilation is a passive process by which air has to pass through an appropriate filter to capture particulate matter and microbes. Photocatalytic oxidation (PCO), has been shown to kill pathogens in air and on surfaces. It is an active process as different types of ions and reactive oxygen species are released into the air where they bind to and eliminate pathogens and pollutants.

Ultimately the objective of air decontamination methods is not to kill all the microorganisms in a room, that would be sterilization, but to reduce the air microbial content and surface contamination in order to lower disease transmissibility.² It is likely that different air decontamination strategies will have complementary and additive effects to make indoor air safer for animals and people.

PCO AND AIRBORNE PATHOGENS

The ROS produced by PCO have been shown to be an effective tool to inactivate different types of enveloped and non-enveloped viruses.^4,12 The principal inactivation mechanisms is by damaging viral capsid proteins from the interaction with ^.OH and ^.O₂^–, followed by the fragmentation of the viral nucleic acid.¹²

The interaction between ROS and bacteria produces damage to the cell wall and cytoplasmic membrane, increasing cell permeability and death.⁴ Interaction with the intracellular Coenzyme A inhibits the respiratory chain, further contributing to microbial death.⁴ Hydroxyl radicals seem to exert the strongest bactericidal activity.¹² Hydroxyl radicals are short lived, particularly unstable and react rapidly with most biological molecules.¹² Hydroxyl radicals and hydrogen peroxide penetrate the cell wall, oxidize membrane fatty acids, induce lipid peroxidation, oxidize proteins, and damage DNA.¹²

Fungi are more resistant to PCO compared to bacteria and viruses, likely due to chitin in their cell wall.⁴ Advance PCO devices have shown good efficacy against fungi in laboratory testing.

SAFETY OF INDOOR AIR

When using an air decontamination device, the comfort and safety for humans and animals should not be compromised. Technologies used in air decontamination must be safe, so environments can remain occupied during the decontamination process.² Indoor air decontamination devices should be able to continuously deal with the fluctuations in indoor air contamination to decrease the risk of airborne pathogens transmission.² Second generation advanced PCO devices, represent a proactive and safe air decontamination method that can be used as an additional layer for infection prevention.    

Volatile organic compounds (VOCs) are a group of gaseous pollutants that cause adverse health effects.¹⁵ Reduction in the concentration of VOCs has been investigated for different indoor environments using, oxidation based processes as one effective mechanism for removal of VOCs.¹⁵ PCO has been shown to decompose VOCs and upon complete oxidation are converted to CO₂ and H₂O₂.¹⁵ A downside of some first generation PCO devices is the production of intermediates like formaldehyde, acetaldehyde and acetic acid, as well as ozone.¹⁵

Breathing ozone is harmful, especially for children, elderly, and people with respiratory problems. Ozone irritates the eyes, nose, and throat; can cause chest pain, coughing, shortness of breath, and throat irritation; and may trigger asthma attacks in those with asthma. Long-term exposure to ozone could cause chronic breathing impairments and compromise the respiratory defense mechanisms.¹⁶ Concerns have been raised that some air decontamination devices emit ozone at rates that are unhealthy.¹⁶ The Occupational Health and Safety Administration states that ozone levels for indoor spaces should be less than 0.1 ppm.¹⁷ The California Air Cleaner Regulation certifies air cleaners (CARB approval) for which the ozone emission is no greater than 0.05 ppm.¹⁸ Underwriters Laboratories (UL) created the validation for zero ozone air cleaning devices (UL 2998). Qualifying zero ozone emission products must demonstrate they emit less than the maximum ozone concentration limit of 0.005 ppm which is below quantifiable level for ozone testing. When purchasing air decontamination devices it is key to assure that they meet safety standards.

CONCLUSION

Veterinarians shifted to a curb-side system to provide a vital service while keeping their team safe. As the pandemic in the US is receding, veterinary hospitals are again allowing pet owners inside the building during the consult. Therefore, implementing strategies to improve the safety of indoor air as part of the infection prevention protocol is vital.

As more air decontamination devices become available, it is important to evaluate its safety and performance. Due to the complexities of indoor air environments, the assessment of microbial survival and evaluation of decontamination methods requires specialized equipment, technical skills, and test protocols.⁸ Today, there are specialized laboratories that worked with sealed aerosol chambers in order to evaluate air decontamination devices in a laboratory setting using microorganism surrogates. When purchasing PCO devices it is key to assured that laboratory testing for efficacy and safety has been done. 

Technologies and strategies aimed to improve indoor air quality must provide a safe, healthy, productive and comfortable environment and reduce energy consumption.¹³

¹ Adamkiewicz G. WHO guidelines for indoor air quality: selected pollutants. In. https://www.euro.who.int/__data/assets/pdf_file/0009/128169/e94535.pdf: World Health Organization 2010:1-484.

² Sattar SA. Workshop on “The Role of Indoor Air as a Vehicle for Human Pathogens”: A Panel Discussion. American journal of infection control 2016;44:S147-149.

³ Sattar SA. Indoor air as a vehicle for human pathogens: Introduction, objectives, and expectation of outcome. American journal of infection control 2016;44:S95-S101.

⁴ Binas V. Modified TiO2 based photocatalysts for improved air and health quality. J Materiomics 2017;3:3 – 16.

⁵ Hay SO, Obee T, Luo Z, et al. The viability of photocatalysis for air purification. Molecules 2015;20:1319-1356.

⁶ Allen JG, Ibrahim AM. Indoor Air Changes and Potential Implications for SARS-CoV-2 Transmission. Jama 2021;325:2112-2113.

⁷ Skowron K. Effcacy of Radiant Catalytic Ionization in Reduction of Enterococcus spp., Clostridioides diffcile and Staphylococcus aureus in Indoor Air. Athmosphere 2020;764:1-12.

⁸ Sattar SA, Kibbee RJ, Zargar B, et al. Decontamination of indoor air to reduce the risk of airborne infections: Studies on survival and inactivation of airborne pathogens using an aerobiology chamber. American journal of infection control 2016;44:e177-e182.

⁹ Sykes J. Canine Viral Respiratory Infections In: Sykes J, ed. Canine and Feline Infectious Diseases Elsevier 2014:170 – 181.

¹⁰ Farhangrazi ZS, Sancini G, Hunter AC, et al. Airborne Particulate Matter and SARS-CoV-2 Partnership: Virus Hitchhiking, Stabilization and Immune Cell Targeting – A Hypothesis. Frontiers in immunology 2020;11:579352.

¹¹ CDC. Ventilation in Buildings In. https://www.cdc.gov/coronavirus/2019-ncov/community/ventilation.html: CDC; 2020.

¹² Bogdan J, Zarzynska J, Plawinska-Czarnak J. Comparison of Infectious Agents Susceptibility to Photocatalytic Effects of Nanosized Titanium and Zinc Oxides: A Practical Approach. Nanoscale research letters 2015;10:1023.

¹³ Zhong L, Haghighat F. Photocatalytic air cleaners and materials technologies. Abilities and limitations. Building and environment 2015;91:191-203.

¹⁴ Allen JG, Waring MS. Harnessing the power of healthy buildings research to advance health for all. Journal of exposure science & environmental epidemiology 2020;30:217-218.

¹⁵ Lee C. Experimental evaluation of in-duct electronic air cleaning technologies for the removal of ketones. Building and environment 2021;196:2-11.

¹⁶ Piazza T, Lee R. Survey of the Use of Ozone-generating Air Cleaners by the California Public. In: California Air Resources Board Research Division; 2006:1-94.

¹⁷ Administration OSaH. TABLE Z-1 Limits for Air Contaminants. In. Occupational Safety and Health Standards: OSHA.

¹⁸ Board CAR. California’s Air Cleaner Regulation. In. https://ww2.arb.ca.gov/about-indoor-air-cleaning-devices-regulation.