Indoor Air Cartoon Journal, April 2019, Volume 2, #54

Measurement of indoor ozone concentration done only when the indoor environment is fully or almost occupied can be misleading. An indoor environment almost or fully occupied can easily cause measured indoor ozone concentration to be well below the recommended limit of 50 ppb, even with outdoor and indoor sources of ozone actively contributing ozone to the indoor environment. Humans are an effective sink of ozone. This is a fact well documented in the literature and still the topic of active scientific investigations. Does this mean human presence is a good strategy for reducing indoor ozone concentration? The answer is definitely no. The skin layers oxidative damage consequence of humans being an effective sink of ozone is noted in the cartoon illustration below. The oxidation products from ozone-initiated surface chemistry causes (oxidative) stress on the skin layers and leads to the damage. Simply put, skin layers oxidative damage is the harm caused to the skin cells and tissues to reduce their natural functions and ability to self repair.

What can be done to prevent or reduce the effect of ozone deposited on building occupants? How to collect relevant data for risk assessment? A good practice would be to first identify the sources contributing ozone to the indoor environment, building condition and operations. Secondly, measure ozone when the indoor environment is under the normal operating condition with the indoor environment almost or fully occupied, and with no occupant. Of course, other indoor air parameters should also be measured. Today’s focus is on ozone and its initiated chemistry products. In addition to the typical measurements in the indoor air, strategies should also be devised to accurately measure ozone-initiated chemistry products near and on human skin, cloth, and nose in order to determine the contribution of human surfaces – skin, hair or cloth. The measurement should be done close to the human surface and decouple from the typical indoor air measurement. Why is this strategy essential for determining the contribution ozone-initiate human surface chemistry on human exposure?

In our typical indoor environment, airborne volatile organic compounds (VOCs), from personal care products or other indoor sources, or those transported from outdoor to indoor are typically high to the extent that they can easily dominate the ozone-initiated chemistry, to generate more oxidation products, in the indoor air than those occurring on human surface, e.g. ozone reaction with skin oil on human surface. This means a significant reduction of ozone in the indoor air, due to human presence, will cause the amount of ozone available to initiate chemical reactions with the VOCs in the indoor air to form oxidation products to be significantly reduced. This phenomenon will cause a significant reduction in the measured oxidation product formed in the indoor air, to the extent that oxidation products generated from ozone-initiated human surface chemistry may not be able to compensate for the reduction.

Despite this, the oxidation products generated from ozone-initiated human surface chemistry can cause damage to human skin and compromise health depending on the duration of exposure, the toxicity of the pollutants, and physiological and psychological vulnerability of exposed human. Therefore, if the measurement of oxidation products formed from ozone-initiated human surface chemistry is not decoupled from that of indoor air, investigators might not be able to appreciate and assess the risk ozone deposition on occupants in a typical indoor environment. How can occupants be helped? Combination of ozone source elimination or reduction, ventilation, filtration or air cleaning strategies will help to reduce exposure.

Do you want to know more about this topic? Read Fadeyi et al. (2013), Thiele et al. (1997), Valacchi et al. (2005), and Wisthaler and Weschler (2010).