The 6 Worst Indoor Air Pollutants

Consider the multitude of air pollutants you could inhale.  You’re probably inhaling some of them right now.  Which of them, though, should you worry about the most?  Well, IAQ researchers have begun addressing that question.  The early work is showing that it’s not very many of them you have to worry about.  In fact, a study published in 2023 concludes that 99 percent of the harm from indoor air pollutants in homes comes from just the six worst indoor air pollutants.

I won’t go into the details of how they came up with their numbers.  But I will give you a general description of what they did.  So before I tell you what those 6 pollutants are, let’s take a look at the concepts of disability-adjusted life year (DALY) and harm intensity.  If you want to dig into the details, I’ve got links to explanations and source material for you.

Disability-adjusted life-year (DALY)

Let’s say someone tells you that wearing a baseball cap backwards is bad for you.  So you go online—perhaps to your favorite AI (Claude is mine)—and see if there’s any truth to it.  And you find out that researchers have looked into this and found statistically valid results.

They categorize the results into two parts:  how much sooner backwards cap wearers die (years of life lost) and how their life was diminished by having this disability before they died (years lost due to disability).  To have any significance to those numbers, they had to get the data for a great number of people.

Then they add those two numbers together.  The years lost plus the years diminished is what’s called the disability-adjusted life-years, or DALYs.  (Although it looks like you’d pronounce this as “day-lees,” I have it on good authority that it’s pronounced “doll-ees.”)

The concept of DALY came from a 1996 paper titled The Global Burden of Disease, a study published for the World Health Organization (WHO) and the World Bank.  (Download link here.)  As you can tell from the involvement of the WHO, the objective was to come up with a way to measure and compare health problems around the world.  (For more, see the DALY entry on Wikipedia.)

Now, back to the topic at hand, the DALY is the starting point for finding the effect of indoor air pollutants.  We know a lot about air pollution and its effects on health.  Particulate matter (PM), for example, is one that’s been studied a lot.  I wrote about a comprehensive paper published on that pollutant last year and also about a study on PM and lymph nodes.

But to compare indoor air pollutants, you need more than just the DALYs associated with different pollutants.  And that brings us to…

Harm intensity

The DALY is helpful in assessing how certain things might shorten or diminish our lives, but it’s based on big-picture analytics.  It doesn’t tell us which pollutants might be the worst offenders.  For example, you could look up the DALYs associated with PM_2.5 exposure and find that it’s 1,536 DALYs per 100,000 people per year.

What’s missing from that number is how the PM_2.5 concentration you’re exposed to would affect your life.  It tells you the level of chronic harm associated with PM2.5 but not how it’s affected by exposure.  And that’s what the authors of this new study added.  From the paper:

Existing IAQ metrics rely on contaminant concentrations but do not directly consider associated health risks. To address this, we introduce the concept of harm intensity (HI) with units of DALY/concentration/person/year, which links chronic harm (DALY/person/year) to the concentrations of airborne contaminants to which people are exposed to [sic].

They used both epidemiological and toxicological data to develop what they call the harm intensity (HI).  That transformed DALY/person/year numbers into DALY/person/year per unit of pollutant concentration.  Most of the pollutants they studied are measured in micrograms per cubic meter (μg/m³).  The two exceptions are radon, measured in bequerels per cubic meter (Bq/m³) and mold, measured in colony forming units per cubic meter (cfu/m³).

Then they used representative concentrations of indoor air pollutants to find the chronic harm caused by the 45 pollutants in their study.  That is, they multiplied the harm intensity by the concentration to find harm.  That brings us back to DALY/person/year, but now with realistic assessments of pollutant concentrations.  (But to make the numbers more palatable, the actual unit is DALY/100,000 people/year.)

And now we can name the worst offenders.

The 6 worst indoor air pollutants in homes

When they ran the numbers, they found that the 6 worst indoor air pollutants in homes are:

1. PM_2.5
2. PM_10-2.5
3. Nitrogen dioxide
4. Formaldehyde
5. Radon
6. Ozone

The graph below shows how much they contribute to the total amount of harm caused by indoor air pollutants.

As you can see, the first two pollutants listed are both particulate matter.  You probably already know that PM_2.5 is particles that are 2.5 microns and smaller.  PM₁₀ is particles that are 10 microns and smaller.  And the one labeled PM_10-2.5 is the group of PM₁₀ particles with the PM_2.5 subtracted number out.

So, particulate matter small enough to cause health problems accounts for 84 percent of the harm.  NO₂ and formaldehyde add 6 percent each.  Then radon and ozone bring up the rear at 2 and 1 percent respectively.  The remaining 39 pollutants account for only 1 percent of the harm.

If you’re wondering about mold and the other bad actors, the table below shows the whole list.  Mold is number 7.

One important pollutant they excluded from this study is carbon monoxide (CO).  It’s not that it’s too low to matter.  They took it out because they claim “its effects are acute.”  Actually, I don’t agree with that because low-level carbon monoxide is more of a chronic than acute problem.  But I think there’s just not enough data on low-level CO to know the chronic effects and levels of morbidity.

The graph and percentages

One thing you may have noticed about the graph above is that the 67% column isn’t even twice as  tall as the 17% column.  And the 17% column is about the same height as the two 6% columns to its right.  There are two things going on here.

First, the graph above uses a logarithmic scale on the vertical axis.  If you’re mathematically inclined, that’s really cool and makes sense here.  If you’re not, showing the data with a linear vertical axis may be easier to understand.  So I asked my friend Claude to do the conversion for me, and here’s what it looks like now:

Second, the percentages on the original graph seem to be incorrect.  They certainly don’t match the numbers in the table.  So I recalculated the percentages and those are the ones you see on the new graph.

The nice thing about the linear-scale graph is that you can see that PM_2.5 really is far and away the most important indoor air pollutant, at least based on the results of this study.

[Hat tip to Kohta Ueno for pointing out that a linear scale would make it easier to see the dominance of PM_2.5.]

Putting this info to use

Let’s see how we can use the layered approach to IAQ I’ve written about to reduce our chances of harm from indoor air pollutants in your home.

Particulates – The first two categories are particles in two size ranges and are easy to deal with.  All you need is good, high-MERV filtration.  Below you can see the PM_2.5 and PM₁₀ in my well-filtered home.  Over the past month, we’ve averaged 4 μg/m³ for each (as measured with an Airthings IAQ monitor), whereas the representative concentrations in the study were 26 μg/m³ for PM_2.5 and 62 μg/m³ for PM₁₀.

Nitrogen dioxide – Consumer-grade IAQ monitors don’t measure NO₂ (or formaldehyde or ozone).  But if you have this pollutant in your house, it’s probably from combustion.  Gas cooktops, gas ovens, and fireplaces would be the main sources.  So to reduce this one, you need a bit of source control.  I’m not saying you have to get rid of your gas range, but the research is pretty clear (here and here) that it’s bad for you.  Barring that, you can reduce the indoor levels with ventilation, assuming the outdoor NO₂ levels are low.

Formaldehyde – This is a volatile organic compound (VOC).  It gets into your house from building materials, furniture, and other stuff you bring in.  It’s used in a lot of different things, so you’ve got to do some research and practice source control and ventilation here, too.

Radon – Radon is a radioactive gas that comes from the ground.  Airtightness and a special kind of ventilation called sub-slab depressurization is what you need to keep the levels of this pollutant low…or just be lucky enough not to have it in your home.  The study used a representative concentration of 78 Bq/m³, which translates to 2.1 picocuries per liter (pCi/l).  The US EPA recommends mitigating if you’re over 2 pCi/l.  I’ve been working on my radon level at home but it’s still too high.  Soon I’ll get a radon mitigation system installed to get the levelbelow 2 pCi/l.

Ozone – Some electronic appliances make ozone.  Think of the smell in the copy room at work.  It’s also something that’s in the outdoor air.  Source control and airtightness would be your go-to methods here…and maybe reducing or turning off the ventilation system when the outdoor levels are high.  And of course, make sure they don’t emit ozone before buying any electronic air cleaner.

The takeaways

Before you start making decisions based on this study, let’s put it into context.  The table above shows the harm associated with 45 different pollutants.  Although the conclusion is that only six of them cause 99 percent of the harm, don’t take the others lightly.  It may well be that toluene, vinyl chloride, or any of the others is causing serious health problems in a particular home.  Remember, that table is based on representative concentrations of pollutants.  Your home has different concentrations of pollutants.

Also, this is a modeling study.  Modeling isn’t bad, but it requires a lot of data as well as followup revisions to the model when the data warrant.  Treat it as a guide.  It’s possible that the 6 worst indoor air pollutants after another five years of collecting and collating data may be different than the ones identified here.

When I wrote about which indoor air pollutants matter most back in 2018, I listed a bunch of them that researchers said were important.  But they didn’t have the kind of quantitative ranking system that this new study gives us.  What we have now may not be the ultimate ranking, but it’s a good start.

All that said, you can’t go wrong by beefing up your filtration, airtightness, source control, and ventilation.  OK, you can, but if you turn your cap around and wear it the right way, you should be able to get it to work out.  ;~)

Hat tip to Professor Bill Bahnfleth for making me aware of this study in his 2025 presentation (pdf) at Building Science Summer camp.

This article was originally published on the Energy Vanguard blog. It was reproduced with permission.