Indoor PM2.5 Under 10: A Noble Goal


window sealing PM2.5 pollution
Window sealing

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.

Dirty pre-filter…

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:

Figure 2. Nonparametric Smoothed Exposure Response Relationship

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:

Life Expectancies in 51 Metropolitan Areas in the US, 1997–2001
Life Expectancies in 51 Metropolitan Areas in the US, 1997–2001

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.

WHO air quality guidelines for annual PM2.5
WHO air quality guidelines for annual PM2.5

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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10 thoughts on “Indoor PM2.5 Under 10: A Noble Goal”

  1. Hi,
    I agree, lower PM2.5 is best.
    Hmm. dirty pre-filter. I have taken care the issue by regular vacuum cleaning of all pre-filters. I have a high quality vacuum cleaner (imported from Europe) which has genuine bag (replaced every 6 weeks), course filter and hepa. I have never seen much visible dirt on pre-filters, perhaps daily vacuum cleaning by ayi helps.
    Another important issue is window and door sealing. Apartment or house cannot be like a bottle, air tight. Unless a filtered supply air from outdoors is available for ventilation, what is the benefit from sealing?

    1. Yes, I was shocked that my pre-filters were so dirty, after just a couple of months! I now know about vacuuming the bottom each week, but I had never heard about this issue before, certainly not from any of the manufacturers, which I find a bit disturbing. And we also just sealed our windows and doors and that should help a lot for better control. But you’re right about not getting too much of a seal, otherwise indoor toxins such as CO2 and VOCs can build up. Definitely need to open the windows for at least 10 minutes on most days just to clean out the rooms.

  2. I feel this post really gives a rather depressing message for us Beijing residents. Especially, the extrapolation of the second figure with Beijing PM2.5 would lead to a quite short lifetime expectancy…

    However, what strikes me is that the life expectancy in China is ~75 despite high pollution (much higher than 25ug/m3!). Furthermore the life-expectancy in Beijing seems significantly larger than 75. In fact, I found ~80 as a cited number ( As a life expectancy of 80 is comparable to the life-expectancy in the regression at the lowest PM2.5, I would be tempted to conclude that PM2.5 has a very negligible effect on life expectancy.

    I feel that a better explanation of the graph is that other factors correlate strongly with PM2.5, which actually are the main cause of mortality (e.g., the more polluted cities in US are the poorer ones and citizens eat worst food/have worst health coverage?), but maybe I missed something simple.

    Anyways, perhaps it would be nice to comment more on the quantitative aspects of the plots (for example, I don’t really understand what is the RR plotted in the first figure…) and thanks for the great blog!

    1. Thanks for the comments. Regarding the graph, one cannot extrapolate that to infinity for any other cities as this study simply cannot prove anything with PM2.5 higher than that data. So one just cannot make assumptions about Beijing from this graph. As I’ve discussed before, the actual relative risk of air pollution is actually quite small, certainly smaller than many other risk factors to health And as I’ve also mentioned before, the best evidence shows that the graphs for cardiac and lung effects from PM2.5 are different, one is linear and one is parabolic … My bottom line is unchanged: air pollution is a well proven risk factor, but many other risk factors have far more effect on our health.

    2. Hi!
      Concerning life expectancy in Beijing, these data deal with people who lived most of their life without the current high rates of pollution, so I guess they are not so relevant for measuring the long-term effects. We would need to know the effective life expectancy of the younger people living now in Beijing (or elsewhere in China), especially the children, but for this we will have to wait for a few decades…
      I think the best we can do is follow reasonable advice like Doctor Saint Cyr’s, while we are in Chinese cities, but also enjoy every occasion we have to go to low pollution areas, like maybe our hometown abroad, or Chinese moutains!

  3. I’m not sure that this 0 pollution is going to be worth it. Because from the ground, the painting, the furniture, I’m almost sure you don’t have 100 % pollution free interior, means you’ll have to deal with other problems, that could be even worst than pollution PM2.5, especially in china where the ingredients for the manufacturing in the building, the interior, furniture etc, is quite hazardous. So I’m not saying which way is right, but I’m myself still looking for the right answer, which is very complex, if you take everything together.

    1. Yes, other indoor air pollutants like VOC, formaldehyde and benzene are also serious concerns for many indoor dwellings here. But a good air purifier also helps filter out much/most of these as well. Most of the same famous PM2.5 air purifiers also work against many of these other chemicals.

  4. I need to get some air purifiers for our home in Delhi. While I’m thankful that the US Embassy here started publishing PM2.5 stats, I am not happy to see how bad they are! Today was an AQI of 274, and it’s only going to get worse with winter coming. The Diwali holiday and all of its fireworks comes at a bad time!

    I’m curious about your opinion of using plants for purifying air. There is a TED talk about it that claims that three specific plants (money plant, mother in law’s tongue, and areca palm) can purify air so well, you could be in a closed bottle with just those three plants and live. I would include the link to the talk, but I don’t want my comment flagged as spam. Anyway, if you were looking for a topic for a future blog post, I would be interested to hear your opinion.

    1. Hi Joanne, I’ve written a few posts about indoor plants, just use my website search engine “plants air pollution” …

  5. Hi! I think that indoor air at 10 µg/m3 or below is absolutely doable, but requires different approach. In my apartment I have designed and built air inlet boxes with HEPA filters from a popular cheap air purifier that replace original non-filtrated inlets. The filter is $10 a piece and actually you could get away by only using this solution.

    I have 6 of them for a flat that is 140sqm. As a result, the air that comes through them is at 1µg or below (that’s the minimum readout on my laser tester). The remaining infiltration is taken care of by two air purifiers.

    Otherwise it was simply not possible to fight the smog that was sucked in to the apartment and I can still take advantage of appropriate air flow for ventilation. Here in Warsaw (Poland) air is not that bad as in China, but definitely one of the worst in EU. At this moment the PM2.5 reading outside is 90 µg/m³ and indoor it’s just 7 µg/m³. I can easily keep it at this level for any outdoor pollution that is possible in Warsaw (that is probably up to 200-250 µg/m³.

    I can recommend that kind of approach for anyone who has a technical possibility to use such solution. Its way better than filling you house with air purifiers, as it will never be as effective!

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