Ageing of indoor air: Causes and implications of exposure and absorbed dose

Indoor Air Cartoon Journal, February 2021, Volume 4, #115

Healthier air is a healthier life. The fresher or cleaner air is, the healthier it is. What is fresh air? What is clean air? Fresh air is a thermally friendly air, new to an indoor environment, which comes from the outdoor and contains no air pollutants or air pollutants at low or negligible concentrations that will not have physiological and psychological effects on the exposed person, irrespective of their vulnerabilities.

Clean air has the same definition as fresh air, except that it does not necessarily have to be transported from the outdoor to the indoor environment. Thus, as defined above, fresh air is clean air, but not all clean air is fresh air. An example of clean air that is not fresh is air taken from an indoor environment, thermally conditioned, treated through filtration and other air cleaning strategies, recirculated, and supplied to indoor air.

The higher the proportion of fresh air in the supplied air to the indoor environment, the healthier the indoor air and environment. Poorly managed fresh and clean air supplied to indoor air will lead to aged air. What is aged air? What is ageing of air? What is age of air? What is the difference between aged air and age of air?

Aged air is the thermally unfriendly air containing air pollutants at a concentration that cause a physiological and psychological effect on an exposed person. At the higher end of aged air, more occupants will be affected negatively.

Ageing of air is the process at which the air thermal parameters and pollutants concentrations are gradually moving towards or in the direction of the unhealthy condition. Age of air is defined in the literature as “average time that air has spent in a zone of the building accumulating contaminants” (Buratti et al. 2011). So, what is the difference between aged air and age of air? How are they linked to ageing of air?

I will use the human analogy to explain better the difference between aged air and age of air. The age of a person is different from an aged person. A younger person might not necessarily be less aged than an older person.

A person that is 40 years old can be aged while a 70 year old person is not aged depending on their exposure, physiological, psychological, social, and economic vulnerabilities and conditions. For example, the 40 year old person, despite the lesser time spent in the world, might have been exposed or consume things that increase the rate of ageing causing him or her to age faster than a 70 year person adopting a healthy life style.

The same thinking can be applied to aged air and age of air. Air with lesser age can be aged, while air with higher age is yet to be aged. It depends on the rate at which air is being contaminated to lead to an unhealthy condition, i.e., the rate of ageing. The rate of ageing will determine when the air becomes aged.

What causes indoor air to age? Air pollutants compromise indoor air quality. The higher the amount of air pollutants being contributed to indoor air per unit time, the higher the rate of ageing of indoor air. Additionally, the higher the rate at which sensible heat and latent heat (moisture) are contributed to or eliminated, depending on geographical location and weather condition, from indoor air in an unhealthy manner per unit time, the higher the rate of ageing of indoor air. Thus, a higher rate of ageing makes aged air occur faster, irrespective of age of air.

Insufficient or no replacement and mixing of clean outdoor air with indoor air, i.e., low or no ventilation, increases ageing of indoor air. Ineffective or no filtration or cleaning of outdoor air and recirculated air supplied to indoor air increases ageing of indoor air. Low or no indoor air movement leads to an increased resident time for air pollutants in indoor air. Thus, air pollutants concentrations can increase significantly due to more time for interactions or chemical reactions between and among air pollutants to produce new air pollutants and cause aged indoor air.

Actions need to be taken to reduce ageing of indoor air by eliminating or reducing the causes. If action is not taken, the risk of implications of health, comfort, and human performance and productivity, especially during the COVID-19 pandemic, will be higher. The presence of aged air (hazard) and vulnerabilities of exposed person, as discussed in a previous article (Vol 3, Issue #113) in this journal, will increase the risk.

The prevention of aged air or reduction of the rate of ageing of indoor air will reduce the vulnerability due to exposure. No exposure means no possibility of being a victim of the harm caused by aged air. The lesser the exposure to aged air due to the prevention and control strategies taken, the lesser would be harm caused to the exposed person within the constraints of their physiological and psychological vulnerabilities.

Lesser harm will occur because of avoidance of or reduced inhalation and absorbed dose of air pollutants in aged air. Absorbed dose is a fraction of inhaled air pollutants retained in the body to potentially cause harm. The toxicity level of air pollutants in aged air could also determine the level of harm caused. Thus, it is imperative to avoid aged air or significantly reduce the ageing rate, especially when the toxic level of potential air pollutants in the aged air is very high, like coronavirus, SARS-CoV-2, causing COVID-19. The use of masks, especially in indoor environments, will also reduce and prevent toxic air pollutants’ inhalation.

Do you want to get more perspective about this topic? Read Batterman and Burge (1995), Buratti et al. (2011), Morawska et al. (2020), and Weitekamp et al. (2020) papers.

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