What Causes Indoor Air Pollution and How to Prevent It


Indoor air pollution sounds so alarming, and rightfully so. The majority of people spend 90% of their everyday lives indoors, so any pollutants in the air can have detrimental effects on health. Even at low concentrations, it can amount to serious exposure over a long period of time.

In this post, we’ll explore what exactly causes indoor air pollution and how to prevent and eliminate it.

Some common “offenders” that contribute to poor indoor quality, or indoor air pollution are:

  • volatile organic compounds (VOCs)
  • carbon dioxide and carbon monoxide
  • radon
  • microbial organic compounds (mVOCs)

Let’s take a quick look at what those mean.


Volatile Organic Compounds (VOCs)

VOCs are typically carbon-chain chemicals that get trapped in our homes. They’re already found in many building materials. Sadly, most of us exacerbate this by continuing to bring in even more sources of VOCs through the everyday products and foods we buy.

Factories use VOCs to essentially bind products, or to give them a certain consistency. VOCs are typically found in wood composite products, adhesives, aerosol paints, paint removers, synthetic building materials, varnishes, cleaning supplies, pesticides, hair spray, and the list continues.

New products contain the highest amount of VOCs. For example, that new car smell you like is a VOC called benzene! A brand-new home is also prone to a high level of VOCs since the building materials are still new. If you’re about to move into a new home and it has new floors, insulation, newly applied adhesives or finishes, or new paint, it might be worth it to clean the air first or to wait a few days before moving into the home.

And even if you have an older home, you still might have a high level of VOCs that have accumulated over time. Not only that, but standards were less stringent in the past, meaning that building materials used in the past might still contain high levels of VOCs.

The good news is that VOCs will disappear over time through the surface of a product in a process called off-gassing. The bad news is that off-gassing can last months and even years, and continuous exposure to VOCs over a long period of time can be dangerous even if one is exposed at low levels.

VOCs in general are known to have respiratory effects, asthma and allergic reactions. Even at low levels, they can be detrimental to the elderly, babies, and fragile pets. Two VOCs in particular, formaldehyde and benzene, are considered as Group 1 cancer-causing carcinogens and should be avoided at all costs at high levels.

Formaldehyde is the most abundantly found VOC in indoor air. It has gained notoriety with the 2015 “60 Minutes” investigation into laminate flooring, and is also the reason why Katrina hurricane victims were found to be sick in cheaply made trailers.

Although authorities are still trying to reach consensus on how exactly individual VOCs should be addressed and measured, there are guidelines. California’s CDPH Section 01350 recommends maximum VOC emission levels of 9 µg m-3  for formaldehyde and 1.5 µg m-3  for benzene. The US Green Building Council recommends a Total VOC (TVOC) concentration of no more than 500 µg m-3. TVOC takes into account the measure of toluene only to try to extrapolate these data as the overall VOC content in the air. It won’t necessarily pinpoint which VOC is off-gassing the most, but it’s a good start.




What Causes It

  • Composite wood products like pressed laminate floors, furniture and kitchen cabinets.
  • Various other building materials like carpet, insulation and adhesives, for example.
  • Mostly anything related to paints or finishes, whether it be the latex paint on the walls or that shiny polyurethane finish on the floors.
  • PVC (polyvinyl chloride) products contain substances called phthalates. Phthalates are known as a semivolatile organic compounds, or SVOCs. These tend to be less volatile than VOC gases, but they accumulate over time on surfaces and mix in with dust.
  • Released from copy and print machines.
  • Can also be caused by occupants in the home:
    • Using deodorant, fragrance and hair spray.
    • Using air fresheners that aren’t natural.
    • Smoking and perpetuating second-hand tobacco smoke.
    • Cleaning with harsh chemicals.
    • Bringing dry cleaned clothes into home.
    • Constantly cooking.


How to Prevent or Eliminate It

  • Know what’s inside your air, plain and simple. Indoor air monitors are becoming increasingly popular and general VOC readings are one of their many functions.
  • Do not live in a home immediately after a wall has been painted, or common building materials like laminate or vinyl flooring or insulation are installed. Wait a few days for most of it to off-gas. Select safer zero VOC paints and otherwise green building materials.
  • Be mindful of what you bring into your home. Limit cleaning supplies to natural ones, choose non-toxic, organic mattresses, be selective in who does your dry cleaning, and limit anything packaged in cheap plastic.
  • If you can afford it, introduce real wood into your home instead of laminate floors, kitchen cabinets or furniture. Those are known to off-gas formaldehyde.
  • Use exhaust fans when cooking in the kitchen. When coloring hair or using hair spray, do this in a well-ventilated bathroom and turn the exhaust fan on there, too.
  • Be mindful of how you freshen the air in your home. It’s easy to get carried away by all the new features seen in commercials, but you need a natural way to remove odors from your home like bamboo charcoal, for example. Remember, it’s better for the air in your home to have no smell at all than to have a fragrance that’s full of VOCs.
  • Throw out VOC-containing chemicals instead of letting them sit on storage shelves and off-gas. Buy smaller, single-use products that you can throw out immediately after use instead.
  • Continuously cross-ventilate the home and use air purifiers.
  • Monitor VOCs with an air quality monitor. If you suspect your home is very polluted and think it will take some time to clear out the air, make sure to get an air quality monitor that lets you store historical data.


Carbon Dioxide and Carbon Monoxide

Most of us are familiar with carbon dioxide. It occurs naturally and is non-flammable. It’s a natural byproduct of respiration by plants and humans alike. It also comes from deforestation and burning of fossil fuels.

ASHRAE states that 1,000 to 1,200 ppm (parts per million) levels of carbon dioxide are acceptable indoors, although ASHRAE’s recommendations tend to be higher than some others. Thus, it’s ideal to stay below 1,000 ppm to avoid the ill effects of carbon dioxide, whenever possible. For reference, 1,000 ppm is about 15 cfm (cubic feet per minute).

Some milder effects like headaches, dizziness, nausea, vomiting, and fatigue will start at carbon dioxide levels above 2,000 ppm. Levels of 5,000 ppm and above pose a serious risk like oxygen deprivation.

However, oxygen deprivation tends to go in hand with carbon monoxide as well.

That’s because carbon monoxide typically occurs in confined spaces where there isn’t enough oxygen. Because of this lack of oxygen, combustion is incomplete and carbon monoxide results instead of the expected carbon dioxide.

Carbon monoxide can be dangerous, as very little is needed to poison you. The higher its levels, the more dangerous carbon monoxide becomes. It’s actually called the silent killer since it’s virtually undetectable with the five senses. It’s colorless and odorless. You may be poisoned by carbon monoxide by steady exposure over time without even knowing it.

Its symptoms include dizziness, nausea, vomiting, shortness of breath, confusion and impaired brain function, blurred vision, and even death. In fact, the CDC states that carbon monoxide poisoning kills 430 people and sends 15,000 people to the emergency room every year in the United States.

As far as safe levels of carbon monoxide are concerned in a home, it depends on how long the exposure lasts. However, it’s generally accepted that even low levels of 70 ppm will cause some mild irritation after some time of exposure.


Carbon Dioxide and Carbon Monoxide


What Causes It

  • Carbon dioxide occurs naturally as a byproduct of respiration.
  • Both carbon dioxide and carbon monoxide are a result of combustion. However, if there is a lack of oxygen, then carbon monoxide will form.
  • Carbon monoxide can form from use of chimneys, furnaces, hot water heaters, leaky gas stoves, dryers, car exhaust pipes, and tobacco products when there is little oxygen in the air.


How to Prevent or Eliminate It

  • Ensure your home has enough oxygen. Keep cross-ventilating the home whenever possible.
  • Routinely have all appliances checked that may give off heat or cause combustion. Check for blockages and make sure your appliances have proper vents so that any carbon monoxide can go outside and won’t accumulate inside your home.
  • Don’t treat your carbon monoxide detector casually. When carbon monoxide levels surge, it’s a matter of minutes or hours before the damage becomes irreversible.
    • Be proactive and change out the batteries on your carbon monoxide detector twice a year and don’t wait for the detector to start telling you the battery is low. Make this a habit by setting it in your calendar.
    • Don’t think that just one detector is enough, especially if you have a large home or multiple floors. You should always have a smoke detector near every occupant’s bedroom so that it can be heard loud and clear while sleeping. Battery-powered detectors are better than just plug-in detectors, in case the power goes out in the home.
    • The alarm in a smoke detector doesn’t necessarily tell you much about the levels of carbon monoxide. It’s possible that it may go off after being exposed continuously for 8 hours at lower levels, but the alarm will sound just as equally when there is a sudden spike in carbon monoxide levels. If you want to be able to understand each incident and put it in context, you’ll need a carbon monoxide detector with digital readings of the exact carbon monoxide levels. A digital reading is also a good way to tell if the detector works before it’s too late.
  • Clean out the soot and creosote in your chimney to prevent carbon monoxide build-up and to protect yourself from fires. If possible, have a professional check your chimney for blockages every year.
  • If you have an attached garage, try to minimize the gases coming out of the vehicle’s exhaust into your home.
    • Most garages nowadays are required to be gas-proofed to try to eliminate this problem, but it’s always a good idea to revisit this idea as your home ages. Check to make sure your garage is well-sealed with caulk, you’re using oil-based paints on the walls, air is being ventilated to the outside instead of into the home itself, etc.
    • Always keep the interior door between your garage and living space closed and make sure this door can thwart off fires and can close on its own. If you just have a standard interior door connecting your garage to the rest of your home, you’re probably at risk. Get a fire-retardant door. If you have a pet, the attached garage is not the place for a pet door.
  • Try to “live outside” during the warmer months. In other words, smoke on your patio and cook food outside whenever possible.



Radon is a radioactive gas that typically seeps through the foundation and foundation walls into a home. It’s usually a concern in homes with basements, although homes with slabs and crawlspaces should be monitored for radon just as much.

Radon is usually found in the soil. It starts as uranium and decays into radon by the time it’s inside the house. Radon then keeps decaying to other substances. For example, it decays into polonium, which enters through the body’s tissues, damages DNA, settles down into the finite branches of the lungs and causes lung cancer. In fact, 14% of lung cancer in the United States is related to radon exposure.

And if you think your home doesn’t have any lead, think again. Radon also decays into lead if it seeps through the foundation into your home.

When trying to assess where your home stands in terms of radon, think about the geology that surrounds your home. For example, limestone is very porous, so radon can seep through. If your area is historically known to have coal or volcanic soil, this is also another source of radon.

The EPA recommends radon testing every year and states that acceptable levels of radon in homes should be below 4 picocuries per Liter (4 pCi/L).

However, radon shouldn’t be treated with a wide brush stroke, either. Radon levels have not only been found to vary across homes of next-door neighbors but sometimes even the readings in a single home can vary.

Radon levels are very sensitive, so things like the way a house was built, the current condition of the foundation and walls and the air flow in the home can have a major impact on how much radon is inside a home.

Because of these sensitivities, close all windows and doors when reading radon levels. Keep the HVAC running as you would normally well before you conduct the test to make sure your tests are realistic. Take your measurements at the lowest level of your home since radon comes up from the foundation. Avoid kitchens and bathrooms, since water tends to give false positives in radon testing.

If radon is found in your home, radon mitigation is a common solution that professionals recommend. Have PVC pipes installed below the slab, run them through the walls and away from the house towards the roof with the help of built-in fans. That allows the gas to escape out instead of entering the basement. This is called sub-slab suction and is the most popular solution for homes with high radon levels. Keep in mind that this has to be used in the less intrusive areas on your home like exterior walls, crawlspaces, unfinished basement areas and attic.




What Causes It

  • Uranium in the soil that has found a way to seep into the home through the foundation and walls.
  • May be found in stone building materials used in the home like granite counter tops, as well as the floor and wall tile in kitchens and bathrooms. Keep in mind that these products don’t typically contain as much radon as is typically seen coming directly from the soil under the home.


How to Prevent or Eliminate It

  • If your home is still in the building stages, monitor what the builder is doing to keep radon out from the very start. Is there a layer of gravel before the slab is laid? Is there a protective sheet to keep soil gases from entering the house? Is there a venting system in place to dissipate the gas from the soil to the outside air?
  • Although a standard radon test will probably work for most existing homes, keep in mind that it may be limited in what it can do. First, you’ll need to mail in the sample and have a lab run the test results for you. Second, it’s only a snapshot of radon levels in one particular area of your home over a period of just a few days.
  • If you suspect radon to be a major concern in your home, then a portable radon detector that gives instant readings might be a good investment. It’s an affordable way to keep measuring radon levels continuously and is a great alternative to the Electret (E-PERM) system that radon professionals have been using for decades (which costs thousands of dollars).
  • If you have the portable radon detector in hand, you can walk around the basement, crawlspace, or slab and test for unusual radon readings. Where in your home could radon be entering?
  • Once you determine that your home has high radon levels, some solutions to consider are:
      • Are there any cracks in the slab or foundation? If yes, fill those in, as necessary.
      • Cover the sump pump if it’s in direct contact to the soil below.
      • If your home is part basement/slab and part crawlspace, check for some possible air leakage in between where the radon might be coming through.
      • Consider a way to depressurize the air in the lower levels of the home to get the already existing air inside the home out. Have a way to control this, though. That way, whenever a wood-burning fireplace is on, carbon monoxide isn’t formed because oxygen is being pulled out of the home.
      • You can have a professional install a radon mitigation system, which essentially is a sub-slab depressurization system. Essentially, the professional would install PVC pipes that pull radon from the foundation. These pipes then divert the radon up and away from the home by using a fan that pulls the air up. More details on this is covered on how to purify air in a basement.
      • If you suspect radon is present in specific pockets inside building materials and the methods above haven’t helped, consider mechanical ventilation. This method simply takes in the outside air to run it through the drywall, in hopes that the radon will dissipate. This method may not work for all homes.



Just like VOCs, microbial volatile organic compounds (mVOCs) are also easily dissipated into the air that we breathe. However, as the name implies, mVOCs come from microbial organisms like bacteria, viruses and mold.

There are over 200 different mVOCs. They can easily be noticed by the musty, strongly pungent odor that they give off during growth. Mold ranges from the common mold that can easily found in bathrooms (known as mildew) to the more serious black mold that most homeowners fear.

These organisms are reliant on plenty of moisture in the environment to survive, so it’s important to understand moisture first before trying to understand mVOCs.

Moisture in a home is measured through relative humidity, or RH. This is simply a measurement of humidity in relation to the maximum amount of humidity that can be taken up within the atmosphere at any given moment in time. This is the same measurement meteorologists use when estimating the likelihood of rain.

But what does all this have to do with our indoor air quality?

Everything! The EPA recommends humidity levels between 30% and 50% in the air inside our homes. Microbial organisms thrive at humidity levels above 60%, and every small increase in humidity causes an exponential difference in their growth.




What Causes It

  • High humidity (RH levels of 60% or above are most ideal environment for growth).
  • Nutrients (organic matter on surfaces, usually cellulose) allow for growth.
  • Areas where water accumulates and condenses.


How to Prevent or Eliminate It

  • Constantly keep your humidity levels between 30% and 50% (30% to 40% is best). Mold growth thrives in humidity levels of 60% and above and does just fine even at 50% humidity. Don’t go below 30%, however. This minimum level is needed, as that’s the minimum necessary for human life and health. A portable dehumidifier should help you achieve the ideal level of humidity.
  • Eliminate the sources of humidity in your home. Are there any leaks in the air duct that are causing water to condense? Are there any leaks in your plumbing?
  • Divert rainwater away from your home. Is the ground around your home sloping away? The EPA recommends at least a 5% slope for the 10-foot perimeter around your home. Consider installing drainage trenches and ensure all door and window openings are well-sealed. Don’t just caulk, add flashing around all windows and doors if your builder hasn’t done it already.
  • If you’re not sure whether your home has been affected by mold, order a mold test. Request a mold test kit that can be mailed in to a lab for tests. Many of those tests include a professional consultation. If mold is noticeable visually, you may not need this test. But if you want to be sure and need a second opinion, it’s worth it.
  • Mold may be inside the walls or in your air ducts. Use a borescope with video camera. The camera functionality typically comes from connecting the device to the camera app on a smartphone.
  • If mold is already present in your home, remember that is needs to be cleaned AND removed. Just because mold is killed doesn’t mean that it still isn’t giving off its toxins. Some things to keep in mind if you already have mold:
    • HEPA filters alone cannot remove mVOCs from the air. Sure, the mVOCs will be trapped in the filters, but you must physically remove them from your home.
    • Cross ventilate your home by opening windows to push the spores outside.
    • Check what might be feeding the mold. It’s common for mold to feed off of the back of wallpaper or off of decomposing matter found in potted soil and indoor plants. Paper, cardboard boxes, natural fabrics, and books also commonly attract mold.
    • If you have mold on walls or hiding inside tight spaces in the bathroom, scrub the surface to remove the mold. For example, a scrubbing brush that attaches to your cordless drill will help you go deeper into the surfaces.
    • Don’t give up when trying to remove mold! Most people think they can clean an area once and get rid of mold for good, but that’s not always true.
      • You may wish to use biocides to kill the mold, although most authorities recommend against it for indoor use due to health risks.
      • If you’d like a more environmentally friendly way to clean off mold, consider a natural cleaner with tea tree oil. It’s been shown to be effective in fighting off mold.
      • You can use safer alternatives, but keep in mind it might be a long process. For example, you can clean with bleach, but you’ll have to constantly keep cleaning until the mold is fully gone since the effects of bleach wear off after 48 hours.
      • Even if you use the disinfectants with the notorious “kills 99.99% of germs” label, it too will wear off after 48 hours. The bottom line is to keep cleaning the area every 2 days until the mold is fully gone. Keep testing the area every few days to see if the mold is coming back.
      • Whichever method you choose to clean up mold, make sure you’re fully protected. Wear gloves and do not touch moldy surfaces with your hands directly. Wear impermeable clothing and a properly fitted respirator. Do not wear a basic face mask.
      • Is there anything you could take outside to clean up and actually kill the spores? If you just have mildew (the white surface mold as opposed to the more serious green and black mold type), simply expose it to natural sunlight. The UV rays are effective at killing mild mold and bacteria. And if this isn’t an option, UV lamps that clean out particles in the air are available, too.
    • Cleaning just the surfaces alone isn’t enough. Viruses are very small and are perpetually suspended in air, almost never settling down on surfaces. If the HVAC is constantly on, the virus will never settle down to a surface and will continue to be suspended in the air.
    • Don’t be afraid to call in the professionals for large areas affected by mold. The EPA defines this as any area that’s larger than 10 square feet. If this is the case and nothing seems to be working so far, your home may need professional mold remediation.



When I first started reading about indoor air quality, the topic became overwhelming and time-consuming very quickly. Instead, information about indoor air quality should be simple and educational. It should encourage you to test the air in your home.

The aim of this post is to give you actionable tips about some commonly found indoor air pollutants in a single resource, from the perspective of an average homeowner. This post is by no means conclusive, but hopefully it’s enough information to inspire you to take action to improve the air inside your home.