More Than Meets the Eye (or Nose): A Q&A with Air Quality Specialist Heidi Rous

More Than Meets the Eye (or Nose): A Q&A with Air Quality Specialist Heidi Rous

As wildfires have plagued California, Oregon, Colorado, and much of our western states in recent years during annual fire seasons, the average citizen has become overwhelmingly more concerned about air quality as it relates to our overall health and well-being.

While a blaze is a real and dangerous threat to one’s safety, poor air quality may have a broader impact and longer health effects. This was made abundantly apparent in the 2020 wildfire season in California, where the air quality index (AQI) reached well over 200, up to 350 or more, resulting in the poorest air quality in the world for many days of the year, surpassing countries with far less stringent pollution control regulations. Weather apps started forecasting “smoke” just as they would “scattered showers” as the poor air quality became so routine that it was vital to the public health to warn people to avoid exposure to the harmful air.

We caught up with our very own air quality, climate change adaptation, and health risk assessment expert, Heidi Rous, CPP, to find out the answers to some commonly asked questions about the sight and smell of smoke and how this situation relates to our health.

Q: Let’s start with the basics. What is AQI? speedometer graphic with the AQI levels
Heidi Rous (HR): The recent fires over the last several years have brought AQI into sharp focus. AQI stands for Air Quality Index and is the daily reporting mechanism that has been in existence for decades, variations of which are used worldwide to clearly communicate public health warnings due to elevated levels of air pollution. As of 2018, metropolitan areas with more than 350,000 people are mandated to report their AQI (which is measured as an average look-back over the last 24 hours), with the option to provide predictions for the next day. In the United States, the AQI uses a six-category scale that allows local officials to be able to succinctly report and advise on health standards and behavior modification recommendations, and the United States Environmental Protection Agency (EPA) provides standardized guidance.

The National Ambient Air Quality Standard (NAAQS) established through the EPA and Clean Air Act states that an AQI of 100 is the safety standard, which is defined as acceptable air quality.

Q: But there were times during the fires in California where the AQI was above 100—even 200—but I could not smell smoke when I was outside. Why is that?
HR: This is a really interesting question. To answer it, we must dive into human biology and our innate ability to detect odors at varying levels, as well as have an understanding of particulate matter and how safety levels are calculated with AQI.

The human perception of odors is complex and is generally thought to involve four major dimensions, including: detectability, intensity, character, and hedonic tone (or relative pleasantness or unpleasantness of the odor). 1 The “detection threshold” is the concentration at which the average person notices an odor, and the “recognition threshold” is the lowest concentration at which the specific odor can be identified. According to research, the difference between these two thresholds “can vary from approximately twofold to tenfold.” 2 The perception of odors is also influenced by the specific concentration, presence or absence of other odorants, and “odor fatigue,” which can be defined as the common temporary experience of losing sensitivity to a specific odor after prolonged exposure to it.

All of this is to say that given the wide variability within the general population and the complex nature of odor detection, it is hard to identify a set of “standard” thresholds for most odorants or pollutants.

Q: Should I be watching the AQI in my area only when I smell smoke?
HR: Well, there is more to particulate matter than meets the eye (or the nose).

Although wildfires can be substantial contributors to ground level concentrations of air pollutants such as fine particulate matter, fires alone are not solely responsible for elevated AQI determinations. In fact, vehicle brake pads, tire wear, diesel exhaust, and emissions from power plants and other industrial processes all contribute to poor air quality, and when you add soot from the fires, only then do you get an accurate representation.
Orange arrow with icons showing air quality, fire, diesel, exhaust, and roadway as air quality pollution contributors

Q: What can I do to help protect myself and my family and remain informed?
HR: Ultimately, as a citizen, your job is to remain vigilant and educated in regard to air quality in your local area by visiting However, you can and should rely on your own senses and experiences to make better decisions. Common sense does apply. For instance, when the AQI is over 200 for PM2.5, your vision is impaired due to smoke, and/or you can smell smoke in the air, do not over exert yourself, remain indoors as much as is possible, replace your home and auto air filters frequently, and if you don’t feel well—slow down.

Rest assured that if your local AQI remains at 100 or higher for a sustained length of time, your local representatives are required to come up with a plan to address this issue, as a matter of public safety.

If risk appears low and there are no indicators of fires nearby, it is appropriate to trust your instincts that air quality is safe to continue your daily routine in your preferred outdoor spaces.

Keep your finger on the pulse of those recommendations, and you can feel confident that you’re keeping yourself and your family safe.

1 EPA, 1992; Reference Guide To Odor Thresholds For Hazardous Air Pollutants Listed In The Clean Air Act Amendments Of 1990
2 Ibid, page 1-8
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