Occasionally, short-term meteorological conditions merge with ongoing human emissions 
to produce extremeoutbreaks of air pollution. In January 2013, a blanket of industrial pollution
enveloped northeastern China. InJune 2013, smoke from agricultural fires in Sumatra 
engulfed Singapore.

In most cases, the most toxic pollution lingers for a few days or even weeks, bringing 
increases in respiratoryand cardiac health problems at hospitals. Eventually the weather 
breaks, the air clears, and memories of foulair begin to fade. But that’s not to say that the
health risks disappear as well. Even slightly elevated levels ofair pollution can have a 
significant effect on human health. Over long periods and on a global scale, such impacts
can add up.

But exactly how much exposure to air pollution do people around the world get? And how 
much health damage isit causing? Since there are gaps in networks of ground sensors, 
University of North Carolina earth scientist JasonWest is leading an effort to answer those
questions using computer models that simulate the atmosphere.

In 2010, West and colleagues published an estimate of the global health effects of air 
pollution based on a singleatmospheric model. More recently, West and colleagues
thought they could improve their calculations by usingresults from a range of atmospheric 
different models—six in all—rather than relying on just one. In 2013, theypublished their 
results in Environmental Research Letters, concluding that 2.1 million deaths occur worldwide
each year as a direct result of a toxic type of outdoor air pollution known as fine particulate 
matter(PM2.5).

The map above shows the model estimate of the average number of deaths per 1,000 
square kilometers(386 square miles) per year due to air pollution. The researchers used 
the difference in pollution levels between1850 and 2000 as a measure of human-caused 
air pollution. Dark brown areas have more premature deaths than light brown areas. Blue 
areas have experienced an improvement in air quality relative to 1850 and a decline
in premature deaths. Fine particulate matter takes an especially large toll in eastern China, 
northern India, andEurope—all areas where urbanization has added considerable quantities 
of PM2.5 to the atmosphere since the start of the Industrial Revolution.

A few areas—such as the southeastern United States—saw PM2.5 concentrations decline
relative to pre-industrial levels (shown in blue). In the southeastern United States, 
the decrease in PM2.5 is likely related to a decline in localbiomass burning that has occurred 
ver the last 160 years.