Alarmingly bad air quality is a perpetual source of worry across the world, whether in developing cities like Beijing and New Delhi or in western capitals like London and Paris. Air pollution is responsible for one in eight deaths worldwide, making it the single largest environmental health risk. But it’s one thing to know that the air is bad, and it’s entirely another to figure out the risks that it poses from day to day.

Most cities now provide an Air Quality Index (AQI) based on the levels of air pollutant levels. Experts are most worried about the level of PM2.5—particles up to 2.5 microns in size—because they can penetrate your lungs and eventually your bloodstream. The problem is that an AQI in itself isn’t very useful; interpretations of various levels vary wildly from country to country, and levels of pollution are now occasionally so high in some cities that they break the AQI scale, which ranges from zero to 500.

Introducing microlives

What if we calculated the cost of pollution in “microlives”? A microlife is equal to about 30 minutes of your remaining life span, so you spend 48 of them per day as a baseline. (A 22-year-old man or 26-year-old woman has about a million microlives, or 57 years, left to live.) Healthy behavior like exercising adds microlives to your overall total; unhealthy behavior—like, say, breathing polluted air—subtracts microlives. The sum total will dictate how long you are likely to live.

We can use the data on the health risks of breathing polluted air to calculate how many microlives it will cost us, as a recent column by David Roberts and Nick Riesland for Project Syndicate recently described.  Quartz’s interactive calculator, above, provides an estimate of how many microlives are lost to a given level of pollution.

How to use the calculator

First, find your city’s PM2.5 level. There are a variety of websites and data sources. AQICN is a good place to start, especially for cities in Asia. AirNow, run by the US Environmental Protection Agency, has extensive data for US cities.

Once you have the PM2.5 number—because the particles are so small, you may get a surprisingly high read-out, even on a fairly clear day—enter it into the calculator to find out how many microlives the air pollution is costing you. (But note thefine print below.)

What does it all mean?

The average pollution levels in Beijing suggest living there will use up about 2 to 3 microlives per day, while levels in New Delhi, one of the world’s most polluted cities, use up an estimated 4-5 microlives, according to calculations by Roberts and Riesland.

How does that compare to everyday activities that can help or hurt your life expectancy?

• Smoking six cigarettes = 3 microlives/day
• Being 5 kg (11 lbs) overweight = 1 microlife/day
• Two hours of sedentary behavior = 2 microlives/day
• Drinking alcohol = 1 microlife for each drink after the first
• Eating one serving of red meat = 1 microlife

There are also many healthy behaviors that can give us extra microlives

• Drinking 2-3 cups of coffee = 1 microlife/day
• 20 minutes of exercise = 2 microlives/day
• Each subsequent 40 minutes of exercise = 1 microlife/day
• Drinking one (but only one) alcoholic beverage = 1 microlife/day

Comparing the cost of air pollution to drinking and smoking may be relevant to expats deciding whether to move to or stay in a city with high PM2.5 levels. Some companies recruiting employees to Beijing, for example, are now considering whether to pay extra to compensate for the high levels of pollution.

How much should they pay? The blog Understanding Uncertainty noted that the UK National Health Service has guidelines stating it will pay £30,000 for medical treatment that will prolong life by one year—that works out to about £1.70 ($2.84) per microlife.

Microlives are a very different calculation for people who have no choice but to live in polluted areas. But families can still use them to gauge the risk of letting their children play outside on a given day, refrain from exercising outside, or use an N95-rated air mask.

The fine print

All of the calculations above are based on some basic assumptions. Most notably, calculating how many microlives a given PM2.5 level will cost you on a given day assumes that you are living your entire life at that level. There simply aren’t enough scientific data on human health and pollution to accurately predict the effect of spending a single day breathing dirty air.