I’ve recently become quite smugly satisfied that my home’s indoor air is always 80% better than the outdoor air, thanks to a quartet of air purifiers working 24/7. But I no longer think my 80% reduction is good enough, and I now have a much more ambitious goal — to keep my home’s PM2.5 concentration under 10-12 µg/m3, all the time — even when the pollution is crazy bad. This target of 10-12 µg/m3 (equivalent to an AQI of 42-50, using the US EPA AQI conversion) may very well be a tilting-at-windmills fantasy, but that is now my goal — backed up by science.
I mention this because my home’s environmental testing team has an indoor target of 10 µg/m3 which is the lowest I’ve heard. Before this, I was more familiar with an indoor air target of 35 µg/m3 (AQI of 100), which is what many testing agencies and air purifier vendors are advising. This 35 may be a fine goal for many, as long as you as an informed consumer realize that chronic exposure to 35 µg/m3 of PM2.5 still leads to long term health problems and is a compromise between economics and health, while under 10 truly is the number where health effects are approaching zero. Perhaps even more importantly, under 10 also is the official recommendation from the World Health Organization. Given all this uncertainty about ideal targets, I thought I’d try to walk my readers through the evidence, and you can come to your own conclusions as to which target you’d like to achieve.
First, there actually is almost no such thing in the real world as a safe level of air pollution. Even with an extraordinarily low PM2.5 under 7 µg/m3 (AQI 30), the data shows an uptick in deaths, cancers and heart disease. As the WHO states in their 2005 WHO Air Quality Guidelines Global Update:
The risk for various outcomes has been shown to increase with exposure and there is little evidence to suggest a threshold below which no adverse health effects would be anticipated. In fact, the low end of the range of concentrations at which adverse health effects has been demonstrated is not greatly above the background concentration, which for particles smaller than 2.5 μm (PM2.5) has been estimated to be 3–5 μg/m3 in both the United States and western Europe.
The WHO updated this guideline in 2013, and with eight more years of research they are even stronger in their assertions:
Thresholds: For short-term exposure studies, there is substantial evidence on associations observed down to very low levels of PM2.5. The data clearly suggest the absence of a threshold below which no one would be affected. Likewise long-term studies give no evidence of a threshold. Some recent studies have reported effects on mortality at concentrations below an annual average of 10 µg/m3.
The WHO Guidelines for Indoor Air Quality (page 4) explain why their indoor air and outdoor air recommendations are the same:
The steering group assisting WHO in designing the indoor air quality guidelines concluded that there is no convincing evidence of a difference in the hazardous nature of particulate matter from indoor sources as compared with those from outdoors and that the indoor levels of PM10 and PM2.5, in the presence of indoor sources of PM, are usually higher than the outdoor PM levels. Therefore, the air quality guidelines for particulate matter recommended by the 2005 global update are also applicable to indoor spaces…
Those italics are mine because this is very important for people to realize: your indoor air goal is the same as the outdoor air goal — and again, that means getting your PM2.5 under 10 µg/m3.
Much of the WHO’s research is based on a couple of famous, very large cohort studies involving hundreds of thousands of people, including the Harvard Six Cities Study and the American Cancer Society Cancer Prevention II Study. These studies show clear increases in death rates from all causes, as well as from heart disease and lung cancers, as air pollution rises. (It’s important to note that all of the data points in these studies, from dozens of cities, had a PM2.5 range from 10 to a maximum of 30 — far lower than most cities in developing countries across Asia now.) All make it very clear that after ~7 ug/m3, the health effects increase. Here’s the graph from the ACS Study:
Below is another graph from another famous article published in the New England Journal of Medicine in 2009, showing how life expectancy in US cities from 1997-2001 decreased with PM2.5 levels above 5 ug/m3:
Because of this and other data, the WHO’s Global Burden of Disease research uses a PM2.5 annual concentration of 7.5 µg/m3 as their counterfactual — the “control” number which would assume to have no health effects. All of their relative risk assessments, including their most recent reanalysis of household air pollution, use 7.5 µg/m3 as the ideal baseline — so why shouldn’t it be our personal goal as well?
Some may still argue that 35 µg/m3 is still the more reasonable goal, as even the WHO officially states that developing countries such as China could use looser guidelines, called Interim Targets. Interim Target-1 states an annual PM2.5 of 35 µg/m3 as the target for annual exposure. Also, this 35 is currently China’s target goal for urban areas (15 for rural areas). And getting under 35 is actually a significant achievement in places such as Beijing, with annual PM2.5 last year of 89.5 µg/m3. But as the WHO states in their Table 1 (below), a level of 35 is “associated with about a 15% higher long-term mortality risk relative to the AQG level” — which again is 10 µg/m3.
The data seems clear to me, and yet here we are in the trenches, still with many differences of opinion. I’m convinced of the science and also have no intentions of waiting years for stronger data and a more unified opinion. Besides, it’s just common sense, isn’t it? Lower is better. For the sake of my wife and new son, I want my home’s indoor PM2.5 under 10 — always. If I can get there, I can literally breathe easier.
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