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 PM2.5 exposure and find that it’s 1,536 DALYs per 100,000 people per year.
What’s missing from that number is how the PM2.5 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:
- PM2.5
- PM10-2.5
- Nitrogen dioxide
- Formaldehyde
- Radon
- 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 PM2.5 is particles that are 2.5 microns and smaller. PM10 is particles that are 10 microns and smaller. And the one labeled PM10-2.5 is the group of PM10 particles with the PM2.5 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 PM2.5 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 PM2.5.]
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 PM2.5 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 PM2.5 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.
Allison A. Bailes III, PhD is a speaker, writer, building science consultant, and the founder of Energy Vanguard in Decatur, Georgia. He has a doctorate in physics and is the author of a bestselling book on building science. He also writes the Energy Vanguard Blog. For more updates, you can follow Allison on LinkedIn and subscribe to Energy Vanguard’s weekly newsletter and YouTube channel.
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hi Alison,
The top exhaust gasses from cooking with natural gas are
Nitrogen Dioxide
Carbon Monoxide
Formaldehyde
Fine Particulate Matter -PM_{2.5}
Benzene
It would be interesting to know what the levels would be after removing the gas stove.
Marklar, I didn’t even install a gas meter on my home. Metered explosive and toxin inducing gas. What a concept.
MARKLAR: Well, they’d have to be lower. But NO2 could still be high. If they had a range hood with good capture efficiency and always used it when cooking, the NO2 due to the range would probably be lower than that created by an unvented gas fireplace that they use every day.
Fantastic post. The cited article appears to have done a great job of creating a novel and intuitively meaningful metric of the risks posed by common pollutants.
What caught my attention the most was where radon placed in the lineup of nasties. A while back you posted a couple of excellent articles on radon exposure and lung cancer. I expressed my frustration in the comments with my perception of the gap between the relatively low robustness of evidence for harms posed by radon and the relatively high prevalence of radon mitigation (sometimes enforced by code). My major point was that we need better research on the threat posed by radon, and that until we have it, homeowners may be misdirecting efforts to improve their IAQ by worrying about radon to the exclusion of other pollutants and/or home efficiency efforts.
The study cited in today’s post puts my concern in better context. If I understand correctly, the typical homeowner would be vastly better served by addressing air filtration compared to radon mitigation. Personally, I don’t know a single person that uses anything but a “rock catcher” air filter (on advice of their “expert” HVAC company), but most of my neighbors and colleagues have paid thousands of dollars to have radon mitigation systems installed (and they are not even code required here). Of course, it would be best for the typical homeowner to address all of the top pollutants referred to in the cited study. But in practice, the typical household would be far better served by spending money and effort on air filtration, air tightness, and ventilation compared to an equivalent expenditure on radon mitigation. You already mentioned the big caveat to my reasoning above: individual homes may have much higher or lower levels of pollutants, including radon.
In my opinion, the big takeaway for building science nerds is: a 4″ filter cabinet and MERV-13 filter filter may be the single most important component for a healthy home. A 4″ filer cabinet is about $150, and MERV-13 filters are about $60 x 3 per year. Compared to other building-science endorsed upgrades, the most effective upgrade may also be among the least expensive.
Ken: Yes, your big takeaway is correct. If you’re spending a lot of money on radon mitigation and still have high PM in your indoor air, you’re doing it backwards. I just updated the article with a new version of the graph that has a linear vertical scale. The logarithmic scale doesn’t really show how much PM2.5 really dominates.
It’s not surprising that the main culprit is particulate matter which is relatively easily filtered out. As it’s a physical irritant in the lungs. Also not mentioned is the indoor humidity plays a large role in the contaminates as well. My neighbor works for one of the commercial air companies and he said their target humidity level in hospitals was 30%-40% Which makes sense to me and it’s also the most comfortable level as well.
I would be interested to see what the differences are in indoor air pollution between N America and the rest of the world that commonly don’t use forced air systems. Countries like Japan heavily invest in air cleaners. They are in every room of houses every hotel room every ferry cabin every hallway and probably even many closets because they don’t have a forced air system to remove air contaminates and they also still use allot of kerosene for heating. One of these days when I visit my friend there I’ll have to bring back an air cleaner and a rice cooker…
When it comes to air filtration the best we seem to do in residential is a 4″ or 5″ filter which is fine if you are pressed for space but still a very poor air filtration solution. There are very simple filter solutions that are far more effective far less restrictive, cheaper and longer lasting than the conventional 4″ filter but you need space for it.
As for the gas range.. If you have proper extraction and make up there is no effect on indoor air quality but in residential that never happens as there are no MUA units available for residential kitchens yet. There are hacked together fan solutions but nothing similar to commercial which you can make a real MUA unit from a few off the shelf equipment but its hard to get them small enough and the cost of building one, space and energy requirements makes them a very high end thing.
I don’t care about Radon for a number of reasons one being that it doesn’t exist in measurable amounts where I am however a few 100 miles to the south it’s common due to the granite there.
I have been looking at air cleaners and there is definitely a need for some sort of standardized testing so you have an idea of what you are getting with units. Most are overpriced for a fan in a box and an often simple filter that is unknown in efficacy. My priority on them is to look at the filter quality and the replacement filter price not the price of the unit.
Robert: Yes, humidity plays an important role in IAQ. It can lead to mold growth, and mold is number 7 on the list. It also can accelerate the offgassing of some VOCs, most notably formaldehyde, which is number 4.
The term “air cleaner” can mean many things, a lot of them bad.
https://www.energyvanguard.com/blog/2-reasons-to-avoid-most-electronic-air-cleaners/
On the issue of air cleaner testing, ASHRAE has a standard for that: Standard 185.3-2024. Method of Testing Commercial and Industrial In-Room Air-Cleaning Devices and Systems for Microorganism Bioaerosol Removal or Inactivation in a Test Chamber. I think this standard might help clean up the field, so to speak.
AHAM has a well-established test standard and certification program for room air cleaners based on the Clean Air Delivery Rate for 3 different categories of particle sizes. I wouldn’t buy a room air cleaner unless it has been certified and rated according this standard.
The chronic long term effects of CO are actually well known – and mostly ignored by the IAQ research in the referenced paper. They made a HUGE mistake.
Carbon Monoxide Toxicology, edited by David Penny “ISBN 9780367398552, 584 Pages, Published September 23, 2019 by CRC Press goes into great detail on the effect on the body, low level chronic impacts and long term health impacts from the low levels that DO NOT trigger CO alarms. Think congestive heart failure, cardiovascular issues, brain injury, etc.
EAPOSTERS (they sell posters for health and homecare workers), sells a poster which does a great job summarizing the long term chronic problems.
www(dot)eaposters(dot)com/store/p351/Carbon_Monoxide_Levels_%26_Health_Risks_Poster_%23150.html
Fireman, wear CO monitoring around fires – as personal safety devices. The alarms will sound at about 30ppm — they need masks!
It’s common for homes with combustion devices that are not direct venting for fresh air and combustion air – to exceed 10ppm -20ppm in the home during various activities. And occupants might not recognize the symptoms. That’s one of the problems noted in the “Carbon Monoxide Toxicology” book – even emergency room doctors miss the symptoms of low level poisoning — unless they’d seen it before.
Ozone is bad, but there is very little most home owners can do in a house EXCEPT lots charcoal activate air filters, frequent replacement of the filters and AVOIDING – Ozone generating devices like UV lights.
Both NOx and Formaldehyde or source control problems… NOx sensors can incorrectly read Ozone levels as NOx – when the Ozone filter on the NOx sensor is depleted.
Not placing CO on the list up before NOx is a mistake. CO Alarms or for immediate health/death alerts. But the steps to eliminate CO will also reduce the NOx in the building.
MOVE CO above NOx.
Dennis: Thanks for the carbon monoxide reference and poster link. I agree with you that CO should be on the list. Whether it is or isn’t, though, it’s something that anyone who has combustion in or around their home should be aware of. Fortunately, low-level CO monitors are available and affordable (https://www.energyvanguard.com/blog/Don-t-Compromise-Get-a-Low-Level-Carbon-Monoxide-Monitor).
One note: Not all UV lights produce ozone. The other problem with them in residential applications is that they’re pretty much never engineered properly. As a result, they don’t do what the installer promised, and they may even damage filters and other susceptible materials in their line of sight.
I think it time for me to check to see if there were updates posted for that original report and ranking of the gases. I believe Max Sherman, PhD had been at the university when the it was written – e.g. I think they thanked him or credited him as an author.
Part of my concern was for days, weeks and months afterwards that paper was used in part as justification why NG cooking should be eliminated world wide. The stats (home cooking) were improperly used with extrapolations of harm that were off the map.
When low level 9ppm CO can lead to congestive heart failure, 15-20ppm low level chronic leading to brain injury, even strokes. That data (EAPOSTERs does given the source – AMA, AHA, etc) the numbers they used can be verified and many of the studies go back 15 and 20 years ago.
I’ve bought and tested some of the “off the shelf CO monitors” that consumers might buy. They are generally worthless. Not calibrated, not reliable. From what I’ve been able to determine – it’s only the IAQ nerds such as yourself, myself and a small group of others (frequently IAQ specialist) that actually take the time to buy and use them. Far too many people depend on CO Alarms as the monitor – and they WILL NOT REPORT chronic low level conditions under 30ppm. That not report is actually part of their UL listing!!!
I have “Zero Air/Dry air” calibration gas and a certified 30ppm CO gas for calibrating sensors. All the gases with documents for their uncertainties – etc. Very few low cost sensors are ever checked! That’s as true with the NOx sensors – most of the bad stuff would be handled with a properly designed HVAC/Ventilation system in the home and a proper kitchen vent hood. That’s something you’ve covered many times. Yet we both know of homes and developers that fail the simple intelligent kitchen hood placement and construction detailing. Architects sometimes install hoods that are essentially recirculating grease filters.
One of the really nuanced things I discovered from the European community – is there strong preference for “plasma kitchen hood filtration systems” – which basically use ‘ozone’ to break down the VOC in the exhaust fumes – which are then filtered out with a activated charcoal filter (ozone likes carbon) and dumped back into the same room. If the occupants don’t change that filter regularly – guess where the ozone ends…
NOx is bad, it has been liked to Parkinson’s and other dementia causes – but cardio-vascular disease has also been liked to a higher rate of Alzheimer’s.
I recently saw a TV report about “third-hand smoke’ which can contain some of those pollutants and I assume nicotine in addition. It recommended screening apartments and houses for previous smokers before moving in. Here’s a current article about it from Mayo Clinic: hhttps://www.mayoclinic.org/diseases-conditions/nicotine-dependence/expert-answers/third-hand-smoke/faq-20057791. My dad heavily smoked from 1952-1995 in this house, and I still find old clothing, fabrics, and walls that exude brownish/greenish/grayish residue or liquid when cleaned. And some rooms still smell of it. I wonder if that can even contribute to high blood pressure.
Debbie: Yeah, houses that have been smoked in for a long time are difficult to clean up. And the hardest and most expensive repair is replacing the drywall. It’s a porous material that sucks up those pollutants from the air. After decades of that kind of exposure, the drywall typically cannot be salvaged.
Hey Allison,
I could live without the multiple references and gratuitous links to Claude, as well as the AI slop image of a ghostly family whose teen son has creepily long fingers. I read what you write because you’re a human expert I trust, and it’s a bit disheartening to see you lauding these tools that lie with absolute confidence. Sounds like you’re interested and enjoying experimenting with them, which is fine, but maybe you can understand that they can be off putting especially to people who might be reading a blog about (among other things) how to conserve energy in the built environment. Just my opinion, but I hope you can take the feedback. Thanks
Thomas: Thanks for your comment. Yes, I can take feedback. I know there are a lot of people using AI in stupid ways. And yes, it definitely needs good human oversight.
But it also is a tool that can make short work of some things. Like taking a graph and changing the scale from logarithmic to linear. I uploaded the graph from the paper and within a much shorter time than it would have taken me otherwise, I got the result you see above. It took a few iterations because it made some mistakes along the way. It’s like a car. Put a bad driver behind the wheel and you can end up in the ditch…or worse.
So yeah, I use AI. But I don’t rely on it to do the thinking. All of the information in my articles and videos is stuff I know or believe to be true. I added “believe” there because sometimes I’m wrong and sometimes the science evolves.
Allison,
As far as I can tell, the paper uses the value of 26 micrograms per cubic meter as a “representative” value for indoor PM 2.5 when they are calculating harm from this pollutant.
Does this number seem plausible to you? To me, it seems extremely high. I only see levels like that on my Airthings meters when there is a lot of smoke in the air from wildfires. I haven’t studied this topic but I am skeptical that median or typical indoor PM 2.5 levels in the US are anywhere near 26.
Jeff
Jeff: I hadn’t thought about the plausibility of it until you brought it up here, but I agree. It does some a bit inflated. The big National Academies overview of particulate matter research says, “the estimated mean indoor PM2.5 concentrations for 2016 was 5.2 μg/m3, and the estimated mean indoor PM10 concentrations for 2014 was 9.7 μg/m3” (p. 118). I’ll see if I can find out more.
You can find a link to the National Academies paper on PM in my What Is PM2.5? article. It’s the last one in the Related Articles section above.
Thank you so much for sharing this article and your insights on the study sited. It filled in a lot of blanks for me. I teach about all the indoor air pollutants to my clients. I know what the published “acceptable exposure levels” are. But now adding the HI to the DALY equation really puts more meaning in where the biggest focus should be when it comes to solving IAQ issues.
I’m now adding you to my “following” list along with Jeffrey Siegel PHD and Dr. Joseph Allen.
After reading several of your articles, I think you might find my filtration system intriguing. Massive surface area, low pressure drop, MERV 16+ (it would be MERV 17 in the old MERV rating system.) Cheers! Dan. essentialairproducts.com/novusaer.