How we estimate pollution exposure

 

The estimates in the State of Global Air draw from the most recent evidence (produced in 2018) as part of the Institute for Health Metrics and Evaluation’s Global Burden of Disease project.

More details on our methodology can be found in the State of Global Air report. Differences from previous years are noted in What’s New?.

 

 

 

How We Estimate Fine Particle Exposure

Fine particle air pollution, commonly referred to as PM2.5, is airborne particulate matter measuring less than 2.5 micrometers in aerodynamic diameter.

Particulate matter concentrations are measured in micrograms of particulate matter per cubic meter of air, or µg/m3.

To estimate global concentrations of PM2.5 in the air, we combined data from:

  • Air pollution monitoring stations (9,960 stations from 108 countries, up from 6000 last year); 
  • satellite observations (from 1998 to 2017); and
  • global chemical transport models.

To estimate PM2.5 exposures for people living in a specific area, we combined:

  • the number of people living within that area; and
  • the PM2.5 concentration to which they are exposed.

This method produces a population-weighted annual average concentration for a given country or region, which is a better estimate of population exposure, because it gives proportionately greater weight to the air pollution experienced where most people live.

 

How We Estimate Ozone Exposure

Ozone is a gas with both natural and human sources. When it is high up in the atmosphere (in the stratosphere), ozone plays a protective role, shielding Earth from harmful rays and ultraviolet radiation. When it is near ground level (in the troposphere), it acts as a greenhouse gas and a pollutant, with harmful effects on human health.

Ozone concentrations are measured in parts per billion (ppb).

To estimate global concentrations of ozone in the air near ground level, we combined data from:

  • monitoring stations (nearly 10,000 sites around the world); and
  • global chemical transport models.

Our assessment focuses on:  

  • measurements taken in the warm season in each country or region, defined as the six months with the highest average ozone levels; and
  • 8-hour daily maximum ozone concentrations during the warm season (the metric used to characterize exposure in the most recent epidemiological studies of ozone’s health effects).

To estimate ozone exposures for people living in a specific area, we combined:

  • the number of people living within that area; and
  • the ozone concentration to which they are exposed.

This method produces a population-weighted annual average concentration for a given country or region, which is a better estimate of population exposure, because it gives proportionately greater weight to the air pollution experienced where most people live.

 

How We Estimate Household Air Pollution Exposure

We report exposure to household air pollution as the proportion of each country’s population living in households where cooking is done with solid fuels. However, this is just the first step in the process of estimating fine particle exposure related to household air pollution. (Since households in colder regions also use solid fuels for heating, this approach likely underestimates household air pollution exposures in those areas.)

To estimate exposure to household air pollution, we combined:

  • information from surveys about cooking practices;
  • information on the age and gender structure of the population in each country; and
  • indoor measurements of household air pollution-related PM2.5.

Unlike previous years, this year we estimated exposure to PM2.5 due to cooking with solid fuels over and above exposure to ambient PM2.5. This step was accomplished by subtracting the estimated ambient PM2.5 exposures from the household air pollution exposure estimates for each location-year of each study in the database. In this way, the analysis provides independent estimates of exposures to household PM2.5 versus ambient PM2.5.