The goal for this post is to give an introduction into the human health effects of air pollution, encourage further discussion, and evaluate an intervention: The use of air purifiers in homes. These air purifiers are inexpensive, standalone devices not requiring any special installation procedure.
This particular intervention was selected out of personal interest, not because I believe it’s particularly effective. It’s plausible that other interventions against air pollution would be much better - for example, providing more people with clean energy for cooking.
We will investigate what it costs to significantly reduce personal exposure to the most damaging form of particulate matter (PM2.5) using these devices. A first analysis suggests that the cost-effectiveness of this intervention is two orders of magnitude worse than the best EA interventions. However, it is still good enough to qualify as an “effective” or even “highly effective” health intervention according to WHO criteria.
The model corresponding to this post can be found here: https://www.getguesstimate.com/models/16649
Epistemic status: I am confident that this post identifies most of the big questions and uncertainties. Given that I have no background in public health, it is possible I’ve missed major pieces of the puzzle, and it’s likely that the specific numbers are off.
Quantifying human health impacts of air pollution
Air pollution is a significant risk factor for cardiovascular disease, cancer, respiratory infections and COPD (WHO). For further discussion, we will look at particulate matter pollution (PM) only - which, by itself, caused a loss of 106 million DALYs (disability adjusted life years) in 2016 - 51% of that in China and India (State of Global Air Report 2018). A Finnish study looked at NO2 and O3 in addition to PM, and found that PM contributes 85% of the total disease burden among those pollutants, with the finest particles (PM2.5, particles with a diameter of less than 2.5µm) producing the vast majority of the harm. Therefore, for the remainder of this post we will only consider PM2.5, which is measured in µg/m³.
Air pollution is a significant problem even in regions with relatively good air: The WHO states that “even in the European Union, where PM concentrations in many cities do comply with guideline levels, it is estimated that average life expectancy is 8.6 months lower than it would otherwise be, due to PM exposures from human sources.”
The role of ultrafine particles
According to some sources, ultrafine particles - which are significantly smaller than 2.5µm - might produce a significant portion of the harm. However, for our investigation here it is enough to consider PM2.5 measurements only, without gathering separate data on ultrafine particles. This is for two reasons: First, we can assume that all kinds of PM are roughly correlated with each other (most literature uses PM2.5 measurements only). Second, the HEPA filter intervention we will suggest is good at filtering ultrafine particles - so if PM2.5 is successfully filtered, ultrafine particles are removed too. 
A large amount of studies investigate the relationship between PM2.5 exposure levels and health effects. The consensus seems to be that this relationship is sublinear when looking at a large range: at high concentrations, each additional µg/m³ of PM2.5 contributes less harm. Here are some studies which examine this nonlinear relationship: 1, 2, 3, 4, 5, 6, 7.
All of those studies calculate extra mortality or rate of illness. This data is not sufficient to estimate years of life lost or DALYs . Other studies do make estimates of these, but they do not take into account the nonlinearity:
- In the EU: an additional 1µg/m³ of PM2.5 for one year, leading to a loss of life expectancy of 0.22 days per person in the EU.
- In India: an average level of of 89.9 µg/m³, leading to a loss of 1.6 years of life per person. 
- In the UK: A loss of life of 2.7 days per person at a level of 9.9µg/m³ for one year (this agrees very closely with the EU values mentioned above).
Indoor vs Outdoor Pollution Levels
Indoor air pollution correlates with outdoor air pollution relatively closely, ranging from “50% lower” to “as high if not higher”, depending on human activity (source). People using open stoves for cooking will experience the highest levels of air pollution - far above the amount of PM2.5 even in bad city air (source). Researchers in Germany measured a ratio between indoor and outdoor PM2.5 concentration of 0.33-0.78 - the lower value for closed windows, the latter value for tilted windows. However those measurements were only performed in an uninhabited room in one building in one city.
The specific ratio between indoor and outdoor PM2.5 concentration might depend on a variety of factors:
- Amount of ventilation through opened windows
- Amount of air infiltration when windows are closed
- Absolute level of outside pollution
- Indoor pollution sources (cooking, smoking, etc…)
- Number of occupants and pets
A comprehensive meta study looked at 61 articles investigating different kinds of buildings. Over 40% of articles found higher indoors than outdoors PM2.5 levels. The authors of the meta study do not attempt to arrive at a consensus for the ratio between outdoor and indoor levels, but we can guess that it's plausible to assume indoor levels equalling outdoor levels (from Figure 3a in the meta study). For smoking households, this assumption is likely to be too low, for non-smoking households with low air infiltration from the outside this might be too high.
For our calculation we don't even need to consider indoor levels explicitely, for reasons described in the next section.
Personal Exposure Levels
The relevant quantity for health effects is the personal exposure a person experiences during a certain period of time. We will not consider this quantity directly. Instead, we will estimate the health effects for certain outdoor PM2.5 levels using real-world data. This data already includes the fact that people spend their days in a variety of indoor and outdoor settings. Therefore, the relationship between outdoor and personal exposure is already implicitly taken into account.
The same can be said for indoor levels, so we don't need to estimate those either.
Difficulties in estimating the effects of PM2.5 reduction
We will propose to place air purifiers in subjects homes, which gives them significantly reduced PM2.5 levels for parts of the day. How can we estimate the health benefits of this intervention? There are two obstacles here.
Obstacle 1: Measurement of mortality vs years of life lost
We know that health impact scales sublinearly with PM2.5 levels. In our summary of existing work, we have seen that most studies that evaluate this nonlinear relationship estimate the mortality risk only and do not attempt to quantify the years of life lost. This is an obstacle to our analysis. In order to arrive at cost-effectiveness estimates, we would very much like to know the additional years of healthy life that can be generated per $ invested in our intervention. The studies evaluated do not provide enough information to calculate that. None of the studies considered calculates the gain in years of life at each PM2.5 level while correctly taking into account the sublinear nature of the relationship.
This could be resolved in several ways:
- Do a proper analysis, incorporating additional data. 
- Take reliable DALY values from one source and multiply it with the shape of the mortality risk curve taken from other sources.
- Use a simple rule of thumb: For example, this UK study suggests multiplying the number of premature deaths by 12 to arrive at the years of life lost (of course, this rule-of-thumb is valid only in the particular case of air pollution).
For our investigation here, we choose neither of these approaches and simply ignore the nonlinearity for reasons that will be outlined below.
Obstacle 2: Is average PM2.5 level the right kind of analysis?
Even if obstacle 1 did not apply and we knew the relationship between average PM2.5 and years of life lost perfectly, this data would not be directly applicable to our analysis. This is because we are not reducing average PM2.5: We are suggesting to place air purifiers in subjects homes, thereby giving them an environment that is much lower in PM2.5 for some portion of the day, and normal levels for the rest of the day. The effects of this on human health might be quite different than simply exposing them to a constant exposure, even if the averages are the same. The effects might be more positive than the calculation based on averages would suggest. 
Finally, outside PM2.5 levels can vary significantly depending on weather and season. Perhaps any analysis based on average PM2.5 levels will always paint an incomplete picture.
Because of the great uncertainties mentioned above, we will assume linear scaling of health effects based on average PM2.5 reductions. We will only use data from one source. That source indicates 0.22 days of life lost from extra exposure of 1µg/m³ for one year, it does not take into account disability. We can be fairly confident in these numbers because they are roughly consistent with the sources from the UK and India.
Air purifiers with HEPA filters for PM2.5 mitigation
A standalone air filtering device, using a HEPA (high-efficiency particulate air) filter, can reduce indoor PM2.5 levels significantly:
- Arrangement: Two devices per participant. One in the bedroom, one in the main living area.
- Device type: Holmes HAP424 costing $80
- Average indoor PM2.5 in control group: 17.5
- Amount of reduction: -60% (table 2 in the source)
- Arrangement: One air purifier per dorm room
- Device type: 3M FAP04 costing $136
- Average indoor PM2.5 before intervention: 96.2
- Amount of reduction: -57%
- https://www.nature.com/articles/7500640 (this study is noteworthy because it investigates many other effects relevant to our discussion. For example, they considered air infiltration rate with closed windows)
- Arrangement: One air purifier in the main bedroom
- Device type: Honeywell 18155, costing $180
- Average indoor PM2.5 without air purifier: between 5.8 and 8.2
- Amount of reduction: -55% in winter, -65% in summer
- Device Type: Samsung AX9000 costing $667
- Average indoor PM2.5 in control group: 4.6 to 12
- Amount of reduction: -29 to -37%
- Note: The study gives a brief review of seven more sources, and summarizes their results as follows: the likely maximum reduction is 57-61% in tightly sealed rooms and 40-55% in real-world conditions.
- Arrangement: one air purifier in each bedroom
- Device Type: Amway Atmosphere
- average indoor PM2.5 before intervention: 62
- Amount of reduction: 72% with mostly closed windows
Calculating the achievable reduction in personal exposure
To estimate the benefits of air purifier use, we need to know the achievable reduction in personal PM2.5 dose - which includes people going about their daily lives and not spending all day at home next to the device.
Terms used here:
- “Personal baseline exposure”: average PM2.5 concentration that a person encounters while going about their daily life, in a variety of indoor and outdoor settings, without using air purifiers
- “Personal mitigated exposure”: average PM2.5 concentration that a person encounters while going about their daily life, while using air purifiers at their home only (not at their workplace or other locations)
In order to calculate the reduction in personal exposure, we will make a simple calculation based on some assumptions. Then, we will compare it with real-world data from studies in which air purifiers were placed in participants homes and personal exposure was measured using portable devices.
Step 1: Simple Model
We assume an air purifier is used in the main bedroom only, and that the bedroom is occupied for 10 hours each day. We also assume that the windows are closed and that the room is fairly small - therefore we will use the more optimistic estimate of a 72% reduction in PM2.5 versus baseline conditions (referenced previously). This yields a total reduction in personal exposure of 30%, or a ratio between personal mitigated and personal baseline of 0.7.
Step 2: Comparison with real-world personal exposure data
We’ll look at studies with people carrying portable measurement devices, and compare them with our calculation of a 0.7 ratio:
- https://www.mdpi.com/1660-4601/16/8/1391/pdf is a simulation study and calculates a large reduction of personal exposure, achieving a 0.28 ratio. This is for hypothetical all-day use and thus seems consistent with our calculations.
- https://www.ncbi.nlm.nih.gov/pmc/articles/PMC6233749/ found a ratio of 0.48 in a senior residential building. They mention that the study participants spend much more time at home than the average citizen (without providing numbers). Therefore this finding also seems consistent with our calculations.
- https://aaqr.org/articles/aaqr-18-11-lcs-0394: They calculate a ratio of Personal mitigated / Personal baseline of 0.71. This matches our guess really closely. They do not mention how much time the study participants spent at home.
Health Benefits delivered by this reduction
With the reduction ratio calculated above, we can calculate the health benefits. As described above, we will assume a linear relationship between average PM2.5 exposure and health effects.
From here onwards, we will consider two locations as examples: Schwechat, a suburb of Vienna, with an average of 15µg/m³ PM2.5, and Muzaffarnaga, India with an average of 89µg/m³ PM2.5 (vales from IQair.com). For the health effects, we use the values from the EU and India studies listed under “existing work” above.
Device and Filter Cost
Air purifiers can cost as little as $80. The studies cited above demonstrate that even cheap devices are sufficient for good results. For device lifetime, we assume 10 years.
Filter Replacement Cost
Performance of the air purifier depends on the condition of the HEPA filter - it needs to be replaced regularly, depending on the amount of particles the filter has already removed. I could not find specific information on how long a HEPA filter lasts, most sources say “about one year”. A test in Beijing revealed a -50% effectiveness drop after 200 days of use, which would mean one year might be too optimistic in high-pollution situations. For our two scenarios of India and Austria, we will assume filter lifetimes of 0,5 years and 1,5 years, respectively.
Air purifiers might use 30-50 Watts (source). In our model, we use local residential electricity prices of our two sample locations, India and Austria.
Putting these numbers together, we arrive at $5230 per DALY for India and $15200 per DALY for Austria (model here).
Placing these numbers in context:
- The Against Malaria Foundation is able to preserve a year of healthy life for $78 - two orders of magnitude better.
- The WHO considers an intervention that costs, per disability-adjusted life year (DALY) avoided less than 3x the national annual GDP per capita “effective”. An intervention that costs less than 1x that amount is considered “highly effective”. Under these criteria, the intervention would be considered “highly effective” in Austria and “effective” in India.
For multiple people living in the same space, cost would go down accordingly. For example, for a bedroom shared by two people, cost per DALY would halve. For a five-person family sharing a small flat, with the purifier placed in the main living area, cost effectiveness might be even better.
Ways to improve the cost-effectiveness
- Homemade devices: An air purifier is basically just a fan and a passive, replaceable filter. It is possible to build a perfectly adequate air purifier with just that. Someone has tested this setup in China and gotten very favourable measurements. However, since good air purifiers are available for $80, homemade devices may not change the calculus much.
- Cheaper HEPA filters. However: according to this blogpost, none exist that are much cheaper yet perform well.
- Targeting the right people: By installing devices in the homes of people who suffer from chronic respiratory disease and who stay home more than the average, we could somewhat increase the health benefits delivered.
- Timing: Pollution varies significantly depending on weather and season. Cost savings might be achieved by using the air purifier only on days of very high pollution. This is questionable though, because of the sublinear scaling of health effects and because HEPA filters degrade based on the amount of particles trapped (so using them only on high-pollution days would not reduce filter cost by much).
- Location: By placing standalone air purifiers in offices or schools, significantly better cost-effectiveness might be achieved.
- Integration with ventilation systems: Ventilation systems of public buildings could suck all incoming air through HEPA filters. This is already done in some places, but not everywhere. I’m unsure how much better the cost-effectiveness would be compared to standalone devices.
- Placement: A HEPA filter can remove more than 99.9% of particles. Therefore, outgoing air from air purifiers is almost completely free from any particles (https://www.tandfonline.com/doi/full/10.1080/02786826.2016.1197375). If someone were to sit or sleep directly in front of the device, it’s plausible PM2.5 levels could be reduced down to almost zero. Possibly a low-powered air purifier, directed at a persons face while they sleep or work, might deliver very large reductions in PM2.5 while requiring very little power. In the summertime, many people use fans for personal cooling - a very cheap intervention would be to strap HEPA filters to those ventilators. This will naturally reduce the cooling effects, but it might be a very cost-effective way to roll out air purification. Furthermore, HEPA filters could be made mandatory for air inlets in cars.
- Power consumption could be improved by building a more energy-efficient device.
HEPA filters are good at removing very small particles. This includes viruses like SARS-CoV-2. It is unclear whether this would have any significant protective effect in practice, if a household is shared with an infected person.
Removal of viruses, bacteria and spores might yield additional positive health effects not already considered in our calculation. Possibly a long-term large-scale air purifier study would be required to measure these effects.
Negative side effects of this intervention
- An air purifier might instill a sense of false safety, for example it might lead people to believe the device offers complete protection against second-hand smoke.
- Noise concerns. In our sample calculation above, we assumed the air purifier is placed in the main bedroom, which could present problems for sleep quality. I haven’t looked into how loud air purifiers are. It is certainly possible to build very quiet air purifiers, because very quiet fans exist, and HEPA filters don’t need particularly high pressure.
... about air pollution in general
- Which other interventions against air pollution could be much more cost-effective? (It seems plausible that the removal of pollution sources would be much more impactful - but also more difficult to achieve - than air filtering)
- How do the cost-effectiveness numbers calculated here compare with existing air quality interventions and organizations, like the Clean Air Task Force?
... on health effects
- How can we quantify the health benefits of a given reduction in PM2.5 more precisely? The main challenge is the pattern of exposure: alternating between low and high. It is plausible to assume that this pattern leads to different health effects than a calculation based on averages would suggest.
- What is the relative health impact of each class of particle sizes? How much of that impact comes from ultrafine particles? HEPA filters might remove ultrafine particles even better than they do particles with sizes around 1µm. If ultrafine particles are what actually causes the worst health effects, it’s plausible that the actual health benefits are greater than what PM2.5 measurements would indicate.
- Are there studies looking at health effects of long-term air purifier use? Some studies measure indicators of cardiovascular health after a short time of air purifier use. Can these numbers be used to estimate the long-term benefit?
… on the practicalities of air purifiers
- In which other ways could the use of air purifiers be supported without direct subvention? (for example, policy interventions)
- There are many potential improvements to the baseline cost-effectiveness of this intervention. What kind of effectiveness could be achieved in a best-case scenario?
- Are there organizations lobbying for air purifiers already? (apart from the manufacturers of those devices)
- Would the use of air purifiers in schools and office buildings be more or less cost-effective than the estimates for homes?
- What’s the best source for cheap HEPA filters and how cheap would that be?
- Which existing air purifier device is cheapest, when taking all operating costs into account? And does it plausibly last 10 years or longer?
- What is the lifespan of a HEPA filter, as a function of hours of use and pollution levels?
- Would it be possible to create an air purifier that’s significantly cheaper to buy and operate than existing models? (building an air purifier is very easy: simply put a HEPA filter on a fan, as explained in this blogpost). By the way: The blogpost was written by Thomas Talhelm, who later founded Smart Air, an air purifier social enterprise. His blog and company website are excellent sources for in-depth discussion of air pollution and filter engineering. Their device (the "Sqair") might be a good candidate for a low-cost, energy-efficient and effective air purifier.
- How would an air purifier need to be designed in order to be easy to use, mostly automatic, and deliver the intervention reliably? Does it need to have a PM2.5 sensor onboard? How would that affect the realistically achievable cost?
Conclusions and Further Work
Air pollution is one of the biggest public health problems of our time. Simple air purifiers are surprisingly effective at reducing the harm. In our sample calculation, the intervention easily meets WHO criteria for a “highly effective” intervention in Austria, and the criteria for an “effective” intervention in India. With just a few small improvements to cost-effectiveness, it would qualify as “highly effective” in India too.
There are many ways in which effectiveness could be improved: If the bedroom is shared by two people, effectiveness doubles. Our calculations were made for 10 hours per day of use. Many people stay home for longer than that, and would correspondingly benefit more from an air purifier in their home. It is plausible that we could find more energy-efficient devices and optimize location, placement and timing. Furthermore, devices could be preferentially given to individuals which are at special risk of pollution-induced illness.
Buying air purifiers for people to place in their homes is probably not a promising EA intervention: Cost-effectiveness is two orders of magnitude worse than GiveWell-recommended charities. That being said, there might be much more cost-effective ways of helping people get access to air purifiers. We might lobby governments to subsidize those devices, or to make HEPA filters mandatory for public buildings and vehicles.
Beyond air purifiers, we could probably find other interventions for mitigating air pollution that are significantly more cost-effective.
I’ve been quite surprised by the results. It seems that using an air purifier has solid health benefits, both in very polluted and in averagely polluted locations. It is surprising that in affluent countries, where people can easily afford these devices, air purifier use is not commonplace. The health benefits are clear and well-studied. I have installed a homemade device in my bedroom, together with a PM2.5 sensor, and plan to place a second device in the office.
If you're interested in air pollution, air purifiers, or would like to collaborate on future research please get in touch.
For their comments and feedback, I'd like to thank Andrés Gómez Emilsson (who previously mentioned HEPA filters in his post on Cause X), Boyang Xia, Cameron Earl, Gernot Ohner, Hannah Metzler, Helene Kortschak, Lorenz Krüger, Matthew Dahlhausen and Matthias Samwald.
Disability-adjusted life years, consisting of years of life lost plus a weighted sum of years spent in disability. ↩︎
With a life expectancy in India of 69 years, this works out to 1.6*365/69/89 = 0.095 days of life lost per year and per extra µg/m³. When compared with the EU values above, which are for a PM2.5 range that is much lower than in India, this demonstrates sublinear scaling of health effects at higher levels. ↩︎
In order to estimate years of life lost, the extra mortality risk is not enough. We would need two additional pieces of information: The age distribution in the population, and the extra mortality risk per age group ↩︎
We have seen that reducing average exposure by x% reduces health effects by less than x%, because of the sublinear scaling. However, in our proposed intervention, we’re not only reducing average exposure, we’re also shifting the distribution of exposure over time: The hours spent at home will be in an environment of greatly reduced PM2.5, the hours spent elsewhere will be at unchanged exposure levels. If we assume that health effects accumulate linearly hour after hour and that the sublinearity of effects applies to each hour individually, this might mean that an average reduction of x%, delivered in this way, would reduce health effects by more than x%. ↩︎