As buildings get tighter, do we risk higher concentrations of indoor air pollutants? That's certainly been the conventional wisdom for a long time. And it's apparently still true for things like radon and VOCs and formaldehyde. Only it turns out these are not always the most prevalent indoor pollutants. Pollutants from outdoors may be the biggest IAQ problem we face.
In a recent pair of blog posts, Allison Bailes has taken on the topic of indoor air pollution in an important way. If you only have time to read one of these posts, read: Which Pollutants Matter Most? in which he picks up on research by Brett Singer of the Lawrence Berkeley National Lab presented at Building Science Summer Camp last August. Singer showed that while most people think about formaldehyde and other VOCs when the term IAQ is brought up, the most common pollutants are incredibly small particulates dubbed PM2.5. These particles are so small that they often are breathed deep into our lungs and often go straight into the bloodstream.
- PM10 : inhalable particles, with diameters that are generally 10 micrometers and smaller; and
- PM2.5 : fine inhalable particles, with diameters that are generally 2.5 micrometers and smaller.
Smoking indoors, burning candles and cooking produces a lot, but most PM2.5 comes from outdoor sources. The exhaust from cars and trucks is one of the most prevalent sources in most areas.
Allison lists the indoor pollutants we should pay attention to, and gives us a succinct description of what we can do about each one to improve indoor air quality. Source control is often the most effective solution, but for the most prevalent pollutants in the PM2.5 range, Allison points to point-source ventilation (kitchen hoods and bath fans to remove particulates) and filtration as effective control strategies.
That brings us to the second of Allison's blogs, 7 Reasons Your Filter Isn't Improving Your Indoor Air Quality. Anyone who has anything to do with installing mechanical ventilation and forced air heating systems should be aware of the issues raised in this blog. It's not just a matter of changing the filter, or choosing a high MERV filter, although that helps. (You need at least a MERV 10 filter to begin getting at least half of the PM2.5 and MERV 13 to filter out a significant portion of the tiny stuff.) Take note, here, that a high-MERV filter may reduce airflow if your system was not designed for it. Allison intends to dive deep into this in the next few blogs.
While filtration is one way to limit exposure of PM2.5, source control still plays a strong role. Given that most PM2.5 comes from outdoors, air sealing is the control strategy for limiting the source. Brett Singer, in his presentation at Building Science Summer Camp, demonstrated how the "penetration factor" - the fraction of outdoor particles coming into a home with infiltrating air - gets lower the tighter a house becomes.
Ventilation, airtightness and IAQ are not for the faint of heart. It's a complex topic that has only been touched on briefly here. JLC has organized a workshop in Austin next month at the Hive for Housing conference where we will continue to explore this complex nexus of physics, chemistry, biology and building systems. Ted Cushman and I will be moderating a discussion with Brett Singer, Kristof Irwin of Positive Energy and Geoff Farrell of Mandalay Homes. Ventilation and IAQ still has a long ways to go before it is as clear to the majority of builders as air sealing the shell is today (and let's face it, a lot of builders are still playing catch up with air sealing).
