Older adults who breathe in very small particles from air pollution for even a day or two are more likely to be hospitalized for many diseases, a recent U.S. study suggests.

Researchers examined particles called PM2.5, a mixture of solid particles and liquid. The particles are too small for the eye to see; each one is about three percent of the size of a human hair or smaller.

PM2.5 can include dust, dirt, soot and smoke.

The researchers confirmed earlier links between short-term exposure to PM2.5 and a higher risk of hospitalization and death from some diseases. Those include heart and lung diseases, diabetes and blood flow blockages in the legs. They also found new links between short-term exposure and more hospitalizations for things like kidney failure and sepsis. Sepsis is a life-threatening illness caused by blood infections.

The study team examined hospital records for Medicare patients around the United States from 2000 to 2012. The researchers chose patients based on their home zip codes.

They looked at data on air pollution levels the day before and day of each patient's hospitalization. The study included 214 different health conditions.

Yaguang Wei is an environmental health researcher at Harvard T.H. Chan School of Public Health in Boston, Massachusetts. He is also the lead writer of the study.

Wei said the most common and dangerous health effects of PM2.5 exposure have been heart and breathing diseases. These are "the leading causes of hospitalization, emergency room visits and even death," he said.

The World Health Organization, or WHO, released air quality guidelines in 2005. The guidelines say people should not be exposed to average PM2.5 levels above 25 micrograms per cubic meter of air for more than 24 hours.

Few studies have looked at the connection between short-term exposure to fine particulate matter and conditions such as sepsis, fluid disorders, kidney failure and intestinal problems.

For such diseases, each cubic meter rise in short-term average fine PM levels was linked with a yearly rise of around 2,050 hospital visits. That cost $31 million in hospital visits, plus costs for care afterward.

Even air pollution levels below safety levels set by the WHO were linked with a higher risk of hospitalizations for conditions such as heart and breathing disorders. These diseases have been tied in earlier studies to PM2.5 exposure.

For these diseases, each cubic meter rise in short-term average fine PM levels was linked with a yearly rise of around 3,642 hospital visits. That cost $69 million in hospital visits, plus costs for care afterward.

Wei and the team of researchers reported their findings in The BMJ medical journal.

Costs related to short-term air pollution exposure are probably far higher than the research suggests, said study co-writer Francesca Dominici. She is also a public health researcher at Harvard.

In an email to Reuters Health, she said the major limit of this study was that costs following hospital visits were not well documented.

People may not be able to avoid exposure to air pollution, but they can still do things to protect themselves, said Matthew Loxham. He is with the University Hospital Southampton in the United Kingdom.

Loxham said in an email that everyone should pay attention to local air quality levels and guidance. But people with heart conditions, asthma, lung disease and other illnesses should pay special attention. Local guidance on poor air quality days may, for example, tell people with such conditions to close windows or avoid some kinds of outdoor exercise.

And, Loxham said, patients and doctors should take note of how poor air quality can worsen health conditions to better understand and treat them.

I'm Alice Bryant.

Lisa Rapaport reported this story for Reuters Health. Alice Bryant adapted it for Learning English. Ashley Thompson was the editor.

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expose – v. to leave (something) without covering or protection

soot – v. a black powder that is formed when something, such as wood or coal, is burned

data – n. facts or information used usually to calculate, analyze, or plan something

zip code – n. a group of numbers that is used in the U.S. as part of an address to identify a mail delivery area

cubic – adj. a measurement that is produced by multiplying something's length by its width and height