Better air quality is the easiest way not to die

What do you worry about more: Getting exercise, eating vegetables, or the air you breathe?

While most things that clearly improve health are well known, one is insanely underrated: Fixing your air. I suspect this is often the most effective health intervention, period. Nothing else is so important while also being so easy to address.

Let’s do a sanity check: Take the four biggest countries in the world and compare how many people died from various causes in 2019.

It’s hard to prioritize health advice. I’m told I should limit salt and eat cruciferous vegetables and do cardio and sleep well and limit alcohol and reduce stress and go for regular checkups. But how much does each of these matter? If you’re a fallible hairless ape, what should you do first?

To answer this, we need numbers. Below, I’ve estimated how much various things impact life through air quality.

This is good news—you can buy extra life with minimal cost in money, time, effort, or willpower!

By all means, control your body-mass, eat well, and start running. Those are important, but they’re also kind of hard. You might fail to lose weight, but if you try to fix your air, you’ll succeed. You should put the stuff with the highest return on effort first, and that’s air.

What I recommend

If you don’t want to read this long, looooooong article (I’m sorry) just do these things in this order:

1. If you have an ultrasonic humidifier, kill it.
2. Monitor local air quality like the weather.
3. No incense.
4. Extinguish candles with a lid.
5. Be careful about smoke when cooking.
6. Get a particle counter.
7. Use an air purifier at home all the time. (Move this to #1 if the outdoor air has high particulate levels where you live.)
8. Install a HEPA cabin air filter in your car.
9. Avoid aerosols.
10. Use a mask very carefully when in dirty air.

A strange list, admittedly, but it’s where the evidence seems to lead.

Particles and the trouble they cause

What we’re measuring

We measure particles in terms of PM_2.5. To understand what these numbers mean, here’s how you could, in theory, measure them:

• Take a cubic meter of air.
• Filter out all of the solid particles.
• Keep only the particles that are 2.5 microns (μm) or smaller.
• Weigh the remaining particles in micrograms (μg).

The units are μg/m³, since you’re weighing particles (in μg) in one m³ of air.

For reference, human hair is around 70 microns wide, bacteria are 1 to 10 and viruses are 0.02 to 0.4. The EPA gives a helpful visualization:

You might ask: Does it matter what the actual particles are? Is 50 μg/m³ created by burning coal or manufacturing cement or natural dust equally harmful? The answer is no (heard of asbestos?) but we don’t really know how much these differences matter in practice.

Quantifying harms

A better measure than deaths is disability-adjusted life years or DALYs. This is the number of years of life lost plus an adjustment for non-lethal conditions that make life worse. For example, schizophrenia is pretty bad, so this measure counts someone who becomes schizophrenic for a year as losing half a DALY. Looking at DALYs lost to different causes gives a similar picture to deaths:

These numbers are only for ambient air pollution, e.g., due to cars, power plants, and manufacturing. Indoor air pollution comes on top of this.

How particles hurt you

We worry about tiny particles because they seem most harmful, particularly in terms of how chronic exposure leads to long-term health problems.

Tiny particles do cause lung cancer, but that’s one of the smaller harms. More than half of harms don’t come from the lungs at all, but from diabetes and cardiovascular disease. Here are the DALYs lost in the US in 2019:

How particles manage to do all this is still a subject of research. The basic story seems to be that tiny particles easily get through the lungs into the bloodstream. These foreign particles then activate your immune system, which sort of goes on a rampage, causing tons of problems.

Why so much cardiovascular disease? Well, you can probably guess where the blood goes after it leaves the lungs.

A heuristic to quantify harms

How much do particles hurt you? While it’s hard to be precise, this section will give two simple heuristics:

• A life-long exposure of 33.3 PM_2.5 costs 1 DALY. This is best for lifestyle changes. For example, moving from somewhere with no particulates to somewhere with a level of 100 costs 3 DALY.
• At 2500 PM_2.5, you lose disability-adjusted life in real time. This is best for one-off events. For example, if you’re exposed to a level of 5000 for 3 hours, you lose 6 disability-adjusted life hours.

Of course, the only way to be sure would be to do randomized experiments where we lock people inside identical environments for their whole lives, vary the particulates they’re exposed to, and see how long they live. Since we can’t do that, we’re left with observational studies.

A 2013 paper looked at life expectancies and particle levels in 545 US counties, while controlling for confounding variables like wealth, smoking, and demographics. They found that each 28.5 μg/m³ of particulates costs 1 year (not disability-adjusted.)

Should we trust this number? Most papers focus on public-health questions, but we can extract estimates. A comprehensive 2017 paper estimates the population mean PM_2.5 in different countries, as well as the health costs of ambient air particles. I took their estimates and multiplied them by the WHO’s life expectancy figures to get an estimate of the total DALYs a person loses over a lifetime. Here we compare these to particle levels.

The straight line shows a fit. It suggests that if you are exposed to 33.3 PM_2.5 for your whole life, you’ll lose around 1 DALY as a result. I wouldn’t put too much faith in this exact number. It varies from country to country, and this is all built upon some very tricky observational statistics. Still, the estimate is reassuringly close the the number from the 2013 paper.

You might ask: Shouldn’t the same level of particles cause more harm somewhere where people live longer since they have more time to spend breathing?

Maybe. I tried relating the particles to the DALY loss in a single year, without multiplying by life expectancy. It suggests that being exposed to 2500 PM_2.5 for one year costs 1 DALY.

Put another way, if you breathe particles at a concentration of 2500, you double the speed at which you move towards your (disability-adjusted) destiny. If you’re exposed to a level of x for h hours, then you lose h × (x / 2500) hours.

In practice, this isn’t too different from the previous estimate since life expectancies don’t vary that much between countries.

Personal exposure

So, we can estimate how much harm particulates do. Your next question should be, how many particles are you exposed to? Probably the answer is ambient levels plus some extra that’s created indoors, but it’s hard to say how large that extra amount is.

Where people spend their time

There are great records of outdoor levels, but people aren’t outdoors very much. Here’s the NHAPS survey on how Americans spent their time from 1992 to 1994.

No recent survey seems to equal this. They even have curves of where people are throughout the day.

Personal vs. outdoor exposure

Are levels indoors the same as outdoors? There’s a strong correlation—particularly if people keep their windows open—but it varies. Typically, levels indoors are higher than outdoors. Chen and Zhao review a bunch of papers that try to estimate the indoor/outdoor (I/O) ratio:

None of this really matters though. Indoor levels vary in space. What you care about is your personal exposure—the air that actually goes into your lungs.

This is hard to study since someone has to carry a measurement device on their person. Still, it’s been done a few times. Early attempts put a small tube near the mouth that passed that air through a filter that was later weighed. More recent studies use devices that measure light scatter.

I couldn’t find any good reviews, so I did my own. Here’s a comparison of the mean personal and outdoor levels in all the studies I found:

Personal exposure is strongly correlated with outdoor levels, but typically a bit higher. If you trust the trendline, it predicts that someone with an outdoor level of 50 would have a personal exposure of 62.5.

However, personal exposure is much less predictable than the graph above would suggest. Each point is averaged over many people. Individual studies that broke things down person by person show a huge range.

So, your exposure is probably ambient levels plus some amount that depends on what you do and where you go. To figure that out, we’ll have to dig deeper.

Particles Outdoors

Most particles are introduced by human activity. Common sources are power plants (particularly coal, but also natural gas and oil), factories, human-made fires, cars, and trucks. Natural sources are dust, wildfires, and (oddly) sea spray.

Variance with respect to city/region- Large: Levels vary hugely throughout the world. Estimates vary, but some countries are really low (New Zealand, Canada) and some are an order of magnitude higher (India, Qatar). Different locations inside of countries are correlated partly because of the same air blowing around and partly because of shared emissions controls. Still, if your city has no wind and lots of factories and cars, levels will be higher.

Variance with respect to location within a city/region- Smallish: How much variability is there between different places in the same city? The answer seems to be some. A massive study in various locations in Europe addressed found that particle levels near streets were around 20% higher than urban measurements which were, in turn, around 20% higher than regional measurements.

Variance with respect to date- Moderate: At the same location, levels vary throughout the year. Here’s the mean concentration in Los Angeles for every day for 20 years, courtesy of the EPA.

You can see the impact of people blowing up fireworks to celebrate their freedom (July 4 every year), everyone staying at home because of a pandemic (March-April 2020), and massive nearby wildfires (September 2020).

Incidentally, how much did those wildfires matter? Many areas saw their levels rise to around 100 for a few weeks and some spiked as high as 200-500. As a pessimistic estimate, suppose your levels rose by 200 for a full month. That raises your yearly average by 16.67, which would cost ½ a DALY if it happened every year. If it just happened once, you’d lose 200/2500=.08 months or 2.4 adjusted life days.

Variance with respect to time of day- Smallish: At the same location and date, levels change by the hour. Manning (2018) combined measurements from 3110 sites around the world to get the following graph.

Here, we have a great riddle: Why are levels lowest in the mid to late afternoon? They suggest that “this remarkable global homogeneity in diurnal PM_2.5 cycles suggests the influence of common factors including the diurnal cycle of mixed layer depth modulated by other processes such as diurnally varying emission patterns.”

What I think this means is this: The sun heats up the earth in the morning. This causes the air near the earth to rise, mixing up the different layers of air. This pulls lots of particles from close to the ground up into the sky, decreasing the density. After the earth cools, the air stops mixing around so much. Also, maybe it has something to do with when people commute and so on.

I’m fascinated by this phenomenon of mixed layer depth but haven’t been able to figure out much about it. Does it change throughout the seasons? Is that why wintertime air quality is often worse? I don’t know.

Particles While Commuting

Driving: Cars generate lots of particles. If you’re driving near lots of other cars particle levels are probably higher than the overall ambient air. A couple of studies I saw found levels in the range of 50-100 in cars. Others suggest it’s a not so bad and similar to just being outside. It probably depends on your car, traffic, local emissions controls, and weather.

Walking, biking, and running: If you’re on a street, your exposure is maybe a bit higher than the background, but not a ton. As mentioned above, street measurements are typically a bit higher than urban measurements. If you’re biking or running, you’re breathing harder. It seems that a typical adult breathes around 15 times per minute but can speed up by a factor of 4 if exercising hard. This might mean that particles accumulate 4 times as fast, but I can’t find any clear evidence.

We could estimate the harms from pollution as a result of running or biking, but I won’t since exercise also improves cardiovascular health, and I don’t know how to calculate the tradeoff.

Riding the subway: Luglio (2020) measured particles for all the major train systems in the northeastern United States. Levels in aboveground stations were always low (10-25).

Smith (2020) found that in the London underground, the Victoria line had a median level of 361, the Northern line 194, a couple other around 50, and the rest even lower. The Victoria line is highest because it is entirely underground, meaning that particles have nowhere to go.

Martins (2015) reviews many previous studies and found levels over 100 in London, Buenos Aires, Paris, Beijing, New York, Stockholm, Shanghai, Barcelona, and Seoul. There were lower levels in Budapest, Guangzhou, Helsinki, Los Angeles, Mexico City, Taipei, and Sydney.

Rant: Let’s face it, these levels are a scandal. Some places are working on it, but progress is slow because it’s hard to retrofit trains. Hear me, transit agencies: Don’t retrofit the damn trains. Just install normal air purifiers in stations. Do this because:

• The problem is that particles build up slowly in tunnels with no place to escape. We can solve the problem at the source by slowly removing particles.
• It’s easy to put static purifiers in stations. Space and power aren’t as much an issue. You can use standard components.
• The particles in trains are coming from the stations and tunnels.
• People also breathe the air in stations.

Are air purifiers too expensive? Well, the New York subway (MTA) has 275 underground stations. Assume pessimistically that each station needs 50 purifiers, and it costs $1000 each per year to operate them. (The MTA is fond of vastly overpaying.) The cost would be 13.75 million per year, less than 0.1% of the MTA’s budget. Seems worth it to avoid exposing millions of people to air five times more hazardous than the most polluted cities in the world.

Particles Indoors

Suppose you’re inside your home. The quality of the air you breathe can be reduced to five factors:

1. Particle levels outdoors.
2. How long particles hang in the air indoors.
3. The exchange rate between indoor and outdoor air.
4. Stuff you do that creates particles indoors (cooking, candles).
5. Stuff you do to remove particles indoors (running a purifier).

We’ve covered #1 already. Let’s do the rest.

How long particles hang in the air

Maybe indoor air is somehow automatically cleaner than outdoor air? Public health guidance to stay indoors during poor air quality suggests this.

Left alone, particles do settle out of the air. With totally still air, this happens deterministically with larger particles falling faster. With real (turbulent) air, particles bounce around until they stick to a surface. This leads to an exponential decay with a rate depending on the particle size and air turbulence. Experiments suggest something like the following:

I have trouble determining how much these times depend on air turbulence or if the stirring rate resembles real-world conditions.

Half-lives of air in homes

Even with all windows closed, air is constantly moving through cracks in the building. Let’s quantify this as a “ventilation half-life”, the amount of time after which half of indoor air has been replaced from outdoors. In typical homes this seems to range from around 1 to 5 hours. It’s longer in more energy-efficient homes and (much) shorter if windows are open.

In summary, unless you actively clean the air, indoor levels are probably similar to outdoor levels plus whatever particles you generate inside. The air is exchanging fast enough that by the time particles have fallen out of the air, new particles have come from outside. (However, closed windows should work for larger particles and there might be some benefit to opening/closing your windows based on changing outdoor levels.)

Exposure with decaying concentrations

To calculate the impact of specific things that generate particles, we’ll need a quick calculation. If you generate a puff of smoke with a peak concentration of c and a half-life of h, levels will fall off slowly over time. What’s your total exposure?

The following graph shows a peak concentration of c=1000 that decays with a half-life of h=0.2 hours. This turns out to be equivalent (same area-under-the curve) to being exposed to a constant of 288 for 1 hour.

Where did 288 come from? The general formula is 1.44 × c × h. This is natural enough: It’s the peak concentration times the half-life times an extra constant because of math. As you’d expect, doubling the peak concentration or half-life doubles the total exposure.

Things that create particles indoors

Smoking: Have you heard? Smoking is bad for you.