Which is better, indoor air or outdoor air?

If you do a quick straw poll on this questions, the results will be mixed. Some point to vehicle exhaust and factories and say the indoor air is better. Others point to volatile organic compounds (VOCs) and airborne microorganisms indoors, and say the outdoor air is better.

This morning I was reading the August edition of the journal, Indoor Air. The editorial by William Nazaroff raised this very question about indoor vs. outdoor air.

The answer to this question has wide implications. If the outdoor air is of a better quality than the indoor air, then we want to promote ventilation (bringing in outdoor air naturally or mechanically). If the indoor air is of a better quality, then we want to only bring in the bare minimum outdoor air as required by code.

When you look across the world, some cities and towns have better outdoor air, and others have better indoor air. But even that statement is an over simplification. We really need to look at individual contaminants. Let me explain by way of illustration…

Let’s say we have a home with elevated VOC levels due to some recent renovations. The home is in an area known to have elevated PM2.5 levels (particulate matter with aerodynamic diameter less than 2.5 microns). Let’s ask the question again… which is better, indoor air or outdoor air? They both have contaminants, so should we open the windows or keep them closed?

The overall answer to the question is the disappointing “it depends”. You will find contaminants in both indoors and out, so “pick your poison”. Before you purchase the next ticket to the moon to escape all these contaminants, let me give you some parting advice:

To reduce indoor contaminants, identify the sources and control them. Why were there elevated VOCs from renovations in our example? Use low-VOC products that are now readily available.

To reduce outdoor contaminants, bring in ventilation through the heating and cooling (HVAC) system. By installing good filtration on the system you will be able to clean and condition the outdoor air before it is supplied to the space. If you are not sure what the outdoor air quality is like in your area, visit this page: EPA AirData.

If you have some thoughts on the quality of indoor vs. outdoor air, please add a comment below!

By Ian Cull

I'm I.A.N. the Indoor Air Nerd. I'm a speaker and consultant on indoor air quality issues. To learn more about me, click "about" at the top of this page.

6 replies on “Which is better, indoor air or outdoor air?”

Great start to a long discussion, Ian. As a Licensed Florida A/C Contractor, I am still amazed that many people do not have regular routine maintenance done on their cooling and heating systems. Inside the A/C unit, it is cold, dark, and wet..a prime breeding place for germs and bacteria. Most of the IAQ issues today are a product of energy efficiency. The battle to cure that issue has led to issues such as Indoor Air Quality. Other means of curing indoor air quality are better filtration, UV lights, Air Purifiers, Humidifiers and Dehumidifiers. Interested parties may visit us at HVAC Parts Online at the address above.

I have always been very curious about the distinction between indoor and outdoor air quality, partly because of the time I had an inspector in my house who informed me that an old drafty house changes it’s air every seven minutes, compared with a tightly sealed house which is every 2 hours. My take on this says that basically, the air indoors is the same as the air outdoors, but with the addition of all the pollution from inside.

Hi Ian,

We recently bought a home in Winnetka (built in the 1960s) and before we moved in we had Scott from your team come do a reading on the VOCs in the home to test the air quality. The home tested well in this regard. The home has a very old furnace and A/C system that will need replacing in the near future. We are trying to determine what is the best way to improve the overall indoor air quality in the home when we do this project. We are considering adding an Energy Recovery Ventilator to draw in some air from outside. We are also considering installing a whole house filtration system to pair with the furnace. We are unsure which of these would be the most beneficial overall. Any thoughts on this would be greatly appreciated! Thank you in advance for your time.


Ventilation and filtration are both important. An ERV is a great option to bring in outdoor air ventilation, although it is not the only way. If budget is a concern, you can always just add an outdoor air duct with a damper at a cost much less than an ERV. As for filtration, I would recommend having the system designed to take a MERV 11 filter or better (MERV is a scale from least efficient: 1, to most efficient: 20). Good luck on the renovation!

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