By Josh Ryan BSc, lab technician at Airmid. Josh has a vast range of experience from microbial cell culturing and aseptic techniques to microbial plating and spectrophotometry.
Reclaiming Fresh Air: The Role of the Open-Air Factor in Healthy Buildings
Sunlight and fresh air in nature play a vital role in generating reactive oxygen species that neutralize airborne pathogens — a phenomenon known as the Open-Air Factor.
Sunlight and fresh air in nature play a vital role in generating reactive oxygen species that neutralize airborne pathogens — a phenomenon known as the Open-Air Factor.
What makes fresh air feel so revitalising? Most of us would attribute it to higher oxygen levels or a cool breeze. However, scientists have discovered something much more powerful in outdoor air: a phenomenon known as the “Open Air Factor.” This natural force helps kill airborne pathogens, improve air quality, and has influenced treatments of diseases like tuberculosis.
The Open-Air Factor (OAF) refers to the antimicrobial properties of outdoor air, first described in the 1960s. Researchers found that outdoor air can rapidly kill airborne bacteria and viruses — a capability that diminishes significantly in enclosed or poorly ventilated indoor spaces. While the exact chemical composition of the OAF remains partly unknown, it is believed to involve a complex mixture of reactive oxygen species (ROS), such as ozone, hydroxyl radicals, and nitric oxide. These reactive compounds are generated through interactions between sunlight (particularly ultraviolet light) and oxygen-containing substances in the atmosphere. They are thought to cause oxidative stress, damaging the membranes and genetic material of airborne microorganisms and effectively neutralising them.
How ROS Are Formed in the Open Air
Reactive oxygen species in outdoor air are produced through natural chemical reactions involving sunlight, atmospheric gases, and biological emissions. Key processes include:
- Photolysis of ozone:
Ultraviolet light breaks ozone (O₃) into oxygen and excited atomic oxygen. The excited atomic oxygen then reacts with water vapour to form hydroxyl radicals (•OH), one of the most reactive species in chemistry. - Sunlight and nitrogen oxides (NOₓ):
Nitrogen dioxide (NO₂) absorbs UV light and splits into nitric oxide (NO) and atomic oxygen, contributing to ozone and ROS formation. - Oxidation of volatile organic compounds (VOCs):
Naturally emitted VOCs, such as terpenes and isoprene from plants, react with ozone and hydroxyl radicals to form peroxides and other reactive intermediates. - Emissions from soil and plants:
Soil microbes and vegetation release compounds that participate in ROS-generating reactions when exposed to sunlight and atmospheric oxygen. - Aerosols and fog:
In moist air, droplets act as microreactors where metal ions (e.g., iron) and peroxides drive Fenton-type reactions, producing hydroxyl radicals.
Together, these mechanisms sustain a low but continuous level of ROS in outdoor environments, contributing to the natural disinfecting power of fresh air.Modern studies have confirmed that outdoor air can rapidly inactivate a wide range of microorganisms, including Mycobacterium tuberculosis, Staphylococcus aureus, Escherichia coli, and influenza viruses. The same pathogens can survive for extended periods in enclosed, poorly ventilated indoor environments. Indoors, the reactive compounds that make up the OAF quickly degrade or are absorbed by walls, furniture, and other surfaces, eliminating the purifying effect.
This has major implications for indoor air quality, especially in tightly sealed, energy-efficient buildings. In our efforts to conserve energy, we have created indoor environments that prioritise insulation and air recirculation over natural ventilation. As a result, we lose the natural cleansing properties of outdoor air, potentially increasing the risk of airborne disease transmission, allergies, and respiratory issues. Reintroducing elements of the OAF into indoor spaces, whether through architectural design, improved ventilation, or air-cleaning technologies, could offer significant health benefits. It may help reduce airborne pathogens and allergens, improve respiratory health, lower infection risks, and even enhance mental wellbeing.
Spending time in fresh air provides measurable health benefits. Studies show that outdoor exposure can improve lung function, reduce inflammation, regulate circadian rhythms, and lower the risk of respiratory illnesses. Simply opening windows more often, using outdoor spaces, or designing buildings that facilitate better air exchange can help bring some of these benefits indoors.
Efforts are currently underway to replicate the effects of the OAF artificially. Emerging technologies like UV-C lighting, advanced oxidation systems, bipolar ionisation, and hydroxyl radical generators aim to replicate the antimicrobial action of outdoor air. While promising, these approaches must be carefully tested and regulated to ensure they do not produce harmful byproducts, such as excessive ozone.
Given these benefits, the challenge becomes: how can we reintroduce the Open-Air Factor into modern indoor spaces?
One promising step is the introduction of ASHRAE Standard 241 — Control of Infectious Aerosols. This new standard marks a pivotal shift in how we design and operate indoor environments to promote health. By prioritizing increased outdoor air ventilation, this standard doesn’t just reduce airborne disease transmission — it also creates an opportunity to bring the Open-Air Factor (OAF) inside our living areas.
The OAF thrives in environments where fresh, unpolluted air is continuously introduced. ASHRAE 241 supports this by requiring higher ventilation rates with outdoor air, helping to dilute indoor pathogens, reduce stagnation, and potentially preserve the beneficial reactive compounds that contribute to the OAF’s germicidal effect. Incorporating these principles aligns with both infection control goals and a broader return to healthier, nature-integrated building design — where the outside air isn’t filtered out, but thoughtfully brought in.
In a world where we spend most of our time indoors, reconnecting with the natural disinfecting power of fresh air, whether through smarter design or simply opening a window, may be one of the most powerful, low-tech health interventions we have.
Why Choose Airmid Healthgroup?
At Airmid Healthgroup, we help bridge the gap between outdoor air science and indoor air solutions. We are a globally recognized authority on indoor air quality (IAQ) and environmental health.
We offer cutting-edge testing services to consumer product manufacturers, including supporting marketing claims and product performance evaluation. Products tested include air purifiers, vacuum cleaners, ventilation systems, building materials, floor coverings, and cleaning products.
We understand that optimizing IAQ can significantly impact human health. With a team of virologists, microbiologists, and environmental scientists, Airmid Healthgroup provides scientifically validated insights that help manufacturers, building managers, and policymakers ensure healthier indoor environments.
Are you interested in how your products perform in improving indoor air quality?
At Airmid Healthgroup, our expert team tests and validates air-cleaning technologies in alignment with ASHRAE 241 and other industry standards.
Whether you’re developing air purifiers, filters, or building materials, we provide scientifically robust testing solutions tailored to your needs.
Contact us today to explore how your innovations can shape healthier indoor environmen
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Keywords
Open-Air Factor, indoor air quality, reactive oxygen species, ASHRAE 241, air purifiers, UV-C, hydroxyl radicals, air cleaning technology, outdoor air disinfection, Airmid Healthgroup, ventilation, antimicrobial air, building health, IAQ, airborne pathogens
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