Outdoor Air Quality Standards: Why They Won’t Work Indoors

Woman sitting in chair with fan turned on indoors

The federal government takes outdoor air seriously. The National Ambient Air Quality Standards (NAAQS) track and regulate 6 criteria air pollutants with specificity and vigor. Since 1970, these standards have done wonders at unburying our cities from the smog disasters of yesteryear.

The success of this program may lead one to wonder why NAAQS isn’t applied indoors. Instead of messing around with the alphabet soup of OSHA, ASHRAE, and NIOSH recommendations, why doesn’t the federal government establish just one standard for all air—inside and out?

It turns out it just isn’t that simple. Below are 5 reasons indoor air quality (IAQ) requires more specific regulations than outdoor air.

1. Respiratory Viruses Spread More Easily Indoors

The 6 criteria air pollutants covered by NAAQS are carbon monoxide (CO), lead (Pb), nitrogen dioxide (NO2), ozone (O3), particulate matter (PM), and sulfur dioxide (SO2). Notice something? None of these categories factor in respiratory viruses.

Admittedly, some pathogens do hitch a ride on particulate matter, and thus NAAQS might be said to account for viruses indirectly. However, researchers now have abundant evidence that COVID-19 spreads through aerosols—that is, microscopic airborne pockets of liquid and viruses—not merely particulates or droplets. Due to some extremely complicated physics, virus-laden aerosols are more likely to infect people in poorly ventilated indoor spaces.

This means that COVID-19 spreads primarily indoors. For instance, a 2021 review of COVID-19 studies found that fewer than 10% of global COVID infections occurred outdoors. Another 2021 study found that “[...]for most cases, the outdoor risk is orders of magnitude less than the indoor risk. Thus, attempts to reduce particulate matter from combustion (such as NAAQS) “will have no effect on the spreading of the disease.”

COVID-19 probably isn’t the only virus with this outdoor/indoor dichotomy. As Wired reports, some researchers are beginning to conclude that “most colds, flu, and other respiratory illnesses must [also] spread through aerosols.” In other words, any IAQ standard that doesn’t factor in viral aerosols is incomplete.

2. Nobody Bleaches the Backyard

Viruses aren’t the only thing missing from ambient air quality standards. Almost every manufactured product exudes volatile organic compounds (VOCs). Since we tend to use said manufactured products inside, indoor air is going to be more susceptible to VOC contamination. New wooden furniture sends out formaldehyde, while nail polish off-gasses acetone. Toilet deodorizers fill the bathroom with paradichlorobenzene, while some perfumes mist benzaldehyde. Especially notable is bleach (sodium hypochlorite), which risks breaking down into a slew of halogenated volatile compounds, especially if mixed with other products.

To be sure, the dose makes the poison; “VOC” is a broad category. While some compounds are dangerous in small amounts, many are merely irritating; others aren’t risky at all until the concentration reaches truly astounding levels. Nevertheless, some people are hypersensitive to VOCs. As a business, you don’t want to exclude such people accidentally by excluding VOCs from your IAQ calculations.

3. PM2.5 isn’t the Worst Particulate Matter

NAAQS categorizes particulates by size: PM10 and PM2.5. PM2.5 is particulate matter with a diameter of 2.5 microns or smaller. PM10 is any particulate matter with a diameter of 10 microns or smaller (and thus technically includes PM2.5). However, PM2.5 itself ought to be divided into 2 categories: PM2.5 and PM0.1—the latter of which is any particle 0.1 microns in diameter or smaller. (PM0.1 is sometimes also referred to as “ultrafine particles.”)

Creating separate standards for PM0.1 (both indoors and out) would be wise, as PM0.1 appears to be an especially dangerous form of particulate matter. It has been linked to heart disease and diabetes in humans, and impaired brain development and cancers in animals. PM0.1 is so small it can even reach the brain.

4. It’s Difficult to Dilute Contaminants Indoors

Imagine if—instead of taking poison in grief over Juliet—Romeo had dumped his tiny vial of poison into a large lake. Would anyone who drank the lake water then be poisoned? Unless he used a military-grade radioactive toxin…probably not. The most likely scenario is that the contaminant would be diluted to the point that it was nigh undetectable.

The same principle applies to ambient versus building air quality. Outdoors—at risk of stating the obvious—is bigger than indoors. Even with adequate ventilation, it's more difficult to dilute contaminants inside, as this video creatively illustrates. Partially for this reason, many indoor contaminants are found at concentrations 2 to 5 times higher than outdoor air. Indoor standards need to consider this fact when setting acceptable exposure levels.

5. NAAQS’s Math is Different

Considering that it is easier to dilute contaminants outdoors, one would think that outdoor standards would be laxer. However, if we compare ambient standards with building recommendations, we see that the outdoor numbers are much stricter. For instance, NAAQS sets a target of 0.075 parts per million (ppm) of SO2, but NIOSH’s recommended exposure limit indoors is 2 ppm—26 times the target outdoor level!

The reason for this is that NAAQS weights their calculations differently, often focusing on the average over a longer period. For instance, the aforementioned NAAQS SO2 level is the “99th percentile of 1-hour daily maximum concentrations, averaged over 3 years” (emphasis ours). Meanwhile, NIOSH’s level is the “time-weighted average concentration for up to a 10-hour workday during a 40-hour workweek.”

The Importance of IAQ Standards

Of the 6.7 million global deaths every year from air pollution, 3.2 million are related to indoor exposure. While most of these occur in impoverished areas (where cooking takes place over a fire or kerosene stove), such a statistic indicates the importance of accurate indoor standards. However, applying the same regulations both inside and outside simply won’t do. Indoor standards require their own unique expertise. ActivePure is eager to share that expertise with all who can benefit from it.

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