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I keep seeing these claims, for example

The indoor air quality is worse than the outdoor air, even in the most industrialised cities.

but I'd believe that air pollution levels are highly variable based on what type of building you're in

Is indoor air pollution really higher than outdoor air pollution? Are there ways to measure it?

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As often, wikipedia has a nice article with quite a lot of links why air quality can be worse inside than outside: en.wikipedia.org/wiki/Indoor_air_quality –  johanvdw Jul 30 '11 at 20:59
Yeah I saw that. But a lot of these only apply to businesses, and not to many modern homes. I certainly don't have a laser printer and don't use whiteout. Also, my home was built in the 1980s so it doesn't have asbestos. Molds don't really grow where I live, and radon gas doesn't really exist in homes built here either. CO2 and Ozone shouldn't even belong in that Wikipedia article (and don't really apply to home environments). So the only thing that's left is the VOCs, which are a bigger concern in offices than in homes –  InquilineKea Jul 30 '11 at 21:41
It depends which pollutants you are measuring. If you measure things like pet dander in homes with pets, then yes. However if you measure pollen and smog, you are going to find more of that outside. –  Chad Aug 1 '11 at 12:59
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migrated from diy.stackexchange.com Jul 30 '11 at 10:13

This question came from our site for contractors and serious DIYers.

2 Answers

up vote 16 down vote accepted

As an environmental chemist, I can honestly tell you that most people don't realize just how bad their indoor air quality is, since they don't realize just how many items in their home are contributing to it.

With regards to measuring indoor air quality, the US National Ambient Air Quality Standards can be found here, and the EPA has a publication database on Indoor Air Quality (IAQ) available here. There are a number of devices used to sample the indoor air periodically which calculate the total amount of chemical/PM present over a given amount of time.

The type of pollution you'll encounter is indeed influenced by where you live, with regards to mold, NOX, SOX and such (the EPA has a lovely "Where you live" resource link available here)

Regional differences aside, there are a few 'standard' pollutants found in pretty much all homes: particulate matter (PM), passive smoke, formaldehyde and other VOC's, air fresheners/cleaners (seriously). It's also worth mentioning heating sources: fireplaces, wood burning stoves, gas stoves, furnaces and the like; if they're not functioning and/or used properly then they can emit pollutants indoors - notably NO2 and SO2.

Home ventilation plays a key role in indoor air quality: if your home isn't well ventilated, then the pollutants are able to accumulate in the ambient air to levels which can trigger a response in the people living there - asthma, headaches, etc. Also, the quality of the materials used in the home can contribute to the quality of the indoor air, as well as its age.

Bernstein, et al (2008) published "The health effects of nonindustrial indoor air pollution" in the Journal of Allergy and Clinical Immunology (121:3) - it's a pay site, but I have access to it, and I'll give a brief overview of their findings below.

Ozone: Links like the one you've provided talk about the 'health benefits' of ozone. They neglect to mention the health risks:

Acute exposure to O3 produces decrements in pulmonary function and exercise capacity and induces airways inflammation in both healthy individuals and those with pre-existing airways disease (ie, asthma, chronic obstructive pulmonary disease). Main sources of indoor O3 are from outdoor O3 resources as well as air purifiers (electrostatic precipitators, negative ion generators, and ozone generators), which are marketed to the public to provide relief from numerous respiratory ailments as well as to reduce odors and destroy microbes. These devices have been shown to increase indoor O3 concentrations in the range of 16 to 453 ppb.

As a sidenote, the 8 hour max ozone exposure is 750 ppb.


Because more than half of all households in the United States use gas, the primary source of indoor NO2 is gas-fueled cooking and heating appliances. An extensive literature has examined the link between NO2 exposure and duration causing adverse respiratory effects in susceptible populations, but results are inconclusive. There is recent good evidence suggesting that children with atopy or asthma, infants who are at risk of developing asthma, and female adults are more sensitive to the respiratory effects of NO2 exposure. Indoor NO2 exposure may also enhance asthmatic reactions to inhaled allergens. In the United States, elevated indoor NO2 levels are more prevalent in lower-income housing developments because of poor ventilation, small apartment size, and frequent use of gas stoves for supplemental heating. Nitrous acid, a product of primary combustion formed as a secondary product of NO2 and other nitrogen oxides and water, is also found on indoor surfaces. Nitrous oxides acidic nature makes it capable of causing respiratory damage leading to respiratory symptoms in patients with asthma at concentrations of 650 ppb over 3 hours.


Sulfur dioxide is a primary combustion product of fossil fuels that can be grouped together with acid aerosols and particles to form a complex group of distinct air pollutants associated with a wide array of adverse health effects, including short-term respiratory morbidity and mortality. Chamber studies have determined that kerosene heaters are the major source of sulfate aerosols and indoor SO2. Some early controlled chamber studies have demonstrated that SO2 can cause bronchoconstriction in healthy adults and adults with asthma, but a more recent study found that SO2 (200 ppb) and its reaction products (sulfuric acid 200 μg/m3 and ammonium bisulfate 2000 μg/m3) caused no significant change in spirometry or symptoms in healthy subjects and subjects with asthma. However, a recently published report examining the effect of indoor heating sources on respiratory symptoms in nonsmoking women reported that a 10-ppb increase in SO2 was associated with increased wheezing and chest tightness


Carbon monoxide is a tasteless, odorless, colorless, and nonirritating gas produced by incomplete combustion of organic material and is the leading cause of poisoning in the United States. The main sources of indoor CO are gas appliances, unvented kerosene heaters, and environmental tobacco smoke (ETS). The main health effect of CO is a result of its ability to impair the oxygen binding capacity of hemoglobin, which can cause headaches, nausea, dizziness, breathlessness, and fatigue, and with high exposures can lead to coma and death. The severity of CO poisoning is dependent on concentration, length of exposure, and the general underlying health status of the exposed individual. Because carboxy-hemoglobin concentrations in blood are cumulative over time, prolonged exposure to low concentrations can result in considerable health effects. Burnett et al found that CO, compared with the other major indoor gaseous pollutants, NO2, SO2, and O3, was the strongest predictor of elderly patients being hospitalized for congestive heart failure.

You mention VOC being more of an office building problem, but that's actually not at all true. There are PLENTY of sources for VOCs in a home - again, you just don't think of them as a 'source' of VOCs.

Bernstein, 2008 includes a nifty table of sources for VOC indoor air pollution:

VOCs and their sources

The above table lists what chemicals are emitted by indoor items, I've taken the time to give a small summary of where you'll find them in your home. Please note that my list is by no means exhaustive of the sources.

Adhesives: furniture, wood floorings, moldings - anything that's been glued in your house. Particleboard also is smothered in adhesive.

Carpeting: it's treated to resist stains and smells - it's often glued and nailed down.

Cleaners: If you're using a cleaner that should be diluted - and don't dilute it properly - then you'll be left with a residue that contributes to the PM pollution in your home.

Linoleum - again, glued down.

Millwork: all that lovely molding, window casings, baseboards and doors - yep. It's contributing.

Office furniture: again, particle board, glues and all. Most homes have at least 1 computer and a computer desk/hutch.

Paints: If your walls aren't painted, then they're most likely wallpapered. Either way - they both emit VOC's.

Others: any spray (air fresheners, deodorants), tobacco smoke, CO & benzene from your attached garage, etc.

Textiles: ALL textiles are treated for shipping - do you wash your drapes before you hang them? What about the pillows on your sofa and the cushions?

Now to answer your question, "is indoor air pollution really higher than outdoor air pollution?"

The answer is: it depends.

The air inside your house comes from the outdoor environment (and the amount present in the outdoor air will vary daily) - therefore any pollutants that were present in the outdoor air are now present in your home.

Koponen, et al (2000) studied the effect of outdoor air pollution on indoor air and determined:

The ventilation rate was also continuously monitored. Indoor particle concentrations were observed to vary from 500 to 104 cm−3 with a high dependence on the outdoor concentrations. This indicates that in this scenario, indoor particles are mainly of outdoor origin. Effects of ventilation rates on indoor air particle concentrations and several inorganic gases were studied. We found that ventilation had a strong influence on indoor particle and gas concentrations. I/O (indoor/outdoor) ratio in different in different particle size classes.

Depending on the outdoor pollution level (PM, smog, NOX, SOX, VOCs) along with the number of indoor source pollutants, it is very likely that the indoor pollution is higher than the outdoor. (This is very much the case in developing countries.)

Again - the ventilation of the home is the major mediator of pollution levels.

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Additionally, carpet captures all sorts of allergens and pollutants. The EPA recommends deep cleaning carpets at regular intervals to help the indoor air quality. –  Wayne Werner Mar 27 '12 at 12:37
Yep, absolutely. I could have kept going on about air pollution sources, but the screaming wall of text would have critted too many people. –  Darwy Mar 27 '12 at 13:10
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Pollution levels are always highly variable based on surroundings.

The adage "the air in your home can be up to five times more polluted than the air outside" has been borne out by scientific study, and if you think about it it's actually quite logical depending on your definition of "pollution".

Most people think of "pollution" as gaseous; harmful organic by-products belching out of cars, power plants and factories. While this is true, a large amount of what has historically been called "pollution" is particulate matter; the soot in smoke from burning wood and fossil fuels for heat, light and eventually electrical energy. Only when VOCs and greenhouse gases were identified as the causes of smog and global warming did the connotation of the term shift towards these gaseous pollutants.

So, by the classical definition, pollution is particulate matter in the air. When that air is relatively calm and still, pollutants don't dissipate and blow away; they instead either settle, or remain suspended in the air. In addition, homes and interior spaces are places where humans spend a lot of time; more so than outside in most industrialized nations. Humans naturally shed skin cells - millions of them a day - which forms a major component of household dust. To this, we add additional particulate matter in the form of perfumes and scents, and in byproducts of steam and smoke when cooking.

In addition, anything present in the air outside the house can also come in. The air inside isn't the same air it's always been since the house was built. So, your indoor air has a good bit of the pollen, smoke, smog and VOCs present in the outside air, on top of the concentrated particulate dust.

Now, most of the statements along these lines of "five times more polluted inside" are part of advertisements for air purifiers. While there's certainly nothing bad (and plenty of good) about having a HEPA filter cleaning the air going through the HVAC system, many of the household purifiers can do more harm than good. "Ionic air purifiers" are particularly bad, because the ionization of oxygen atoms causes the production of ozone (O3); an O2 molecule (or the water vapor in the air) will split into O- ions, and those ions will attach to other oxygen molecules and settle into a stable covalent structure. While ozone's great up in the stratosphere where it absorbs UV rays, it's toxic to most living things, a major component of photochemical smog, and you don't want it in your house. The same ionization process can also produce nitrogen-oxide compounds like NO, N2O and NO2 when the diatomic nitrogen that comprises 70% of the atmosphere becomes involved. None of these are good gases to be breathing on a daily basis either. So, while an Ionic Breeze does indeed remove dust and particles from the air, it also produces far more harmful "pollution" than it removes.

As far as measuring it, there are tools designed to capture particulate matter in an air sample and identify what type and how many particles are in that sample. They're mainly used for testing after an asbestos or mould abatement project. There are also detectors for gaseous pollutants such as carbon monoxide, radon and ozone. These are all rather specialized tools that you wouldn't normally have access to as an average joe.

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Hi Keith and welcome to Skeptics.SE. I appreciate you posted this answer when the question was at diy.stackexchange.com; thank you for that. Now that it has been migrated to Skeptics, we have some different community standards, including providing some references to support your claims. For example, when you say it something has been borne out by scientific studies, you should give examples. (There are quite a large number of claims here, sorry!) –  Oddthinking Jul 30 '11 at 13:54
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