Indoor Air Quality

Indoor air quality is important for our health because we spend so much time indoors.

What is Indoor Air Quality?
Building Science and Indoor Air Quality (IAQ)
Ventilation and ACH
VOC and CO₂
Using CO₂ as a proxy for exposure to SARS-CoV-2
Indoor Air Quality in Portland Public Schools
Air Quality Index (AQI)

What is Indoor Air Quality?

“Indoor Air Quality (IAQ) refers to the air quality within and around buildings and structures, especially as it relates to the health and comfort of building occupants. Understanding and controlling common pollutants indoors can help reduce your risk of indoor health concerns.”

“Health effects from indoor air pollutants may be experienced soon after exposure or, possibly, years later.”

Also according to the EPA:

“Americans, on average, spend approximately 90 percent of their time indoors, where the concentrations of some pollutants are often 2 to 5 times higher than typical outdoor concentrations.”

“People who are often most susceptible to the adverse effects of pollution (e.g., the very young, older adults, people with cardiovascular or respiratory disease) tend to spend even more time indoors.”

“Indoor concentrations of some pollutants have increased in recent decades due to such factors as energy-efficient building construction (when it lacks sufficient mechanical ventilation to ensure adequate air exchange) and increased use of synthetic building materials, furnishings, personal care products, pesticides, and household cleaners.”

Since it has a huge impact on our health and quality of life, it’s a good idea to look for ways to understand, monitor and improve the quality of the indoor air we breathe.

Building Science and Indoor Air Quality (IAQ)

Building science is the multidisciplinary body of knowledge related to the physical and human elements of our built environment. More concretely, building science is the study of energy use, indoor air quality and human and animal comfort in and around homes, public buildings and industrial spaces.

People who design and manage buildings use building science to help them make choices about the construction and operation of human-made physical spaces we use and inhabit. Historically, building science focused on energy consumption and human comfort. For example, an engineer uses knowledge about human comfort to specify the amount of insulation in the walls, the type of heating and air-conditioning system used, and the thermostat set points that control the heating and air-conditioning.

Concerns about energy costs lead architects and engineers to find ways to reduce energy consumption while maintaining comfort. Focusing only on energy consumption encourages us to minimize the amount of outdoor air brought into our living space, but that is ultimately not good for indoor air quality.

As we’ve learned more about human comfort and health, the quality of the indoor air has become more prominent in the design and operation of buildings. Increasing the use of outdoor air helps (usually!) improve the quality of indoor air by reducing exposure to indoor pollution. The exception is when the outdoor air quality is bad, for example during severe forest fires, or for buildings with air intakes near heavily used freeways.

Ventilation and ACH

Although there are many factors affecting energy use and indoor air quality, a critical factor is the amount of fresh air (or outside air) introduced into a building. Ventilation rate is the rate at which fresh air is introduced to a room or building.

Ventilation results in an exchange of air: adding fresh air from the outside requires the removal of an equal amount of air from inside the building or room. When there is a significant temperature difference between inside and outside, increasing the ventilation rate increases the energy consumption. For example, in winter the cold outside air needs to be heated before (or after) it is introduced into the living space and the energy embodied in the warm air that is displaced is lost. Therefore, ventilation and energy conservation work in opposite directions. Increasing the ventilation rate improves the indoor air quality, but it also increases the energy consumption necessary to maintain comfort in the living space.

A common way to express ventilation rate is in air changes per hour or ACH. An energy efficient “tight” house designed a decade or more ago might have only 0.5 ACH, which means that in one hour an amount of air equal to half the volume of the house is replaced. Scientists studying human health in indoor environments recommend ACH higher than 0.5. If the indoor air is recirculated through high efficiency filtration and other air-cleaning processes, the amount of fresh outdoor air can be reduced.

Thought experiment: how is indoor air quality managed on space ships and the extraterrestrial space stations?

Note that the ACH value refers to the amount of air entering and leaving a space. Suppose you had a window and a door open to your room resulting in an ACH of 2. That doesn’t mean all the air in the room is replaced twice every hour. The fresh air entering through the window will mix with the air in the room. Because of mixing, any indoor pollutants are reduced gradually due to ventilation. Increasing the ventilation rate causes any pollutants to decrease more quickly, but an ACH of 2 does not mean all the pollution would be gone in half an hour.

VOC and CO₂

Two common substances that affect IAQ are volatile organic compounds (VOC) and carbon dioxide (CO₂). There are many other important chemicals that affect IAQ, but we will focus on VOC and CO₂ as we learn about indoor air quality.

VOCs are a class of compounds that contain carbon and want to exist as gases in air. The term “organic” in VOC does not refer to grown without artificial ingredients. Rather, the organic in VOC is from chemistry and refers to molecules with carbon in covalent bonds with hydrogen. Humans are filled with organic chemicals, even if we eat a lot of junk food.

The “V” in VOC refers to the ease with which a chemical compound evaporates. Think of the smell of fresh paint or nail polish remover or rubbing alcohol. The chemicals in those substances easily evaporate and mix in the air and they easily register with the nerves in your nose. Not all VOCs are harmful, at least in small concentrations. For example, the smell of a rose is due to the VOCs it emits. The problem is that VOCs from human made solvents, e.g. paint thinner, benzene and acetone, are known to cause adverse health reactions. In other words, many human-made VOCs can make you sick.

CO₂ is a natural substance that we expel when we breathe. Our lungs allow us to add oxygen and remove CO₂ from our blood. In small concentrations, like that in clean outdoor air, CO₂ is harmless. Outdoor air has a CO₂ concentration of about 450 ppm, where ppm means parts per million. Typically (depending on exertion levels) according to sciencing.com the air we exhale has a CO₂ concentration of 40000 ppm!

“Humans, and many other species, need air to live. They breathe in the combination of elements and compounds and exhale a similar set with different proportions. Exhaled air consists of 78 percent nitrogen, 16 percent oxygen, 4 percent carbon dioxide and potentially thousands of other compounds.” Source.

Doing the math:

Equation to convert 4 percent CO2 to ppm

The high concentration of CO₂ in the air we exhale air means that staying in an enclosed space with minimal ventilation will cause the CO₂ level in that space to increase. Adding more people to a given space means that the CO₂ levels will rise faster.

Although CO₂ at 450 ppm is benign, CO₂ at 1000 ppm makes us groggy and interferes with our ability to concentrate and think. That explains why sitting in a stuffy (read poorly ventilated room) makes it hard to pay attention to a teacher. And you thought teachers are boring! It’s the low quality air that puts you to sleep!

Using CO₂ as a proxy for exposure to SARS-CoV-2

SARS-CoV-2 is the virus that causes COVID-19. A primary mechanism for transmitting COVID-19 is by inhalation of SARS-Cov-2 from aerosols, which are small particles suspended in the air.

There is currently no easy way for real time measurement of the concentration of SARS-CoV-2. However, we can use the CO₂ level as an indirect indicator of potential harm from contaminated air using the following reasoning.

Humans exhale CO₂.
In spaces with low ventilation rates and/or large numbers of people, CO₂ levels tend to rise.
Elevated CO₂ levels means that you may be re-breathing air exhaled by another human.
Re-breathing air from someone with COVID-19 (even asymptomatic COVID-19) increases your risk of contracting COVID-19.
Therefore, maintaining lower CO₂ levels in shared spaces is a way to mitigate the risk of COVID-19 transmission by reducing the possibility of re-breathing air containing SARS-CoV-2.

Note that, in general, it is not harmful to breathe air in a room with other humans when adequate ventilation or air-cleaning is provided. In other words, re-breathing itself is not bad. Re-breathing air with suspended SARS-CoV-2 is bad.

According to the CDC:

Limited information exists regarding a direct link associating CO2 concentrations to a risk of COVID-19 transmission. Changes in CO2 concentrations can indicate a change in room occupancy and be used to adjust the amount of outdoor air delivered. However, CO2 concentrations cannot predict who has SARS-CoV-2 infection and might be spreading the virus, the amount of airborne viral particles produced by infected people, or whether the HVAC system is effective at diluting and removing viral concentrations near their point of generation

Also note that in a room full of people who do not have COVID-19, the level of CO₂ will rise, but the CO₂ level itself does not cause transmission of COVID-19.

Perhaps it is easier to understand the role of CO₂ measurement in this video by Nicole Wetsman on the Verge Science YouTube channel that explains the link between SARS-Cov-2 and indoor air quality.

In summary, we can use CO₂ levels as an indirect measure the amount of re-breathing of air exhaled by other people. The CO₂ level concentration alone does not measure the concentration of SARS-CoV-2. Rather, the CO₂ level is a proxy (a stand-in, a comparison for) for the potential for transmission by breathing air containing SARS-CoV-2.

Put more simply: if our indoor spaces have low CO₂ concentrations, there is lower potential for re-breathing air from others, and therefore a reduction in the mechanism for transmitting COVID-19.

Completely separate from the issue of COVID-19 transmission, spending time in indoor spaces with lower levels of CO₂ will help us stay more alert and healthy.