Older adults who are exposed to tiny particles in air pollution for just a day or two are more likely to be hospitalized for a wide variety of common health problems, a U.S. study suggests.

Researchers focused on so-called PM 2.5, a mixture of solid particles and liquid droplets smaller than 2.5 micrometers in diameter that can include dust, dirt, soot and smoke. They confirmed previously-known links between short-term exposure to PM 2.5 and an increased risk of hospitalization and death from heart and lung diseases, diabetes, and clots in the large veins of the legs. They also found new links between short-term exposure and increased hospitalizations for conditions ranging from sepsis to kidney failure.

The study team examined hospital data for Medicare patients nationwide from 2000 to 2012. They focused on 214 different health conditions, and looked at data on average air pollution levels the day before and the day of each hospitalization based on patients' home zip codes.

"We discovered several previously unknown but common diseases among older adults, such as fluid and electrolyte disorders, septicemia, anemia, urinary tract infections, and renal failure, even when daily PM2.5 concentrations were below the current World Health Organization (WHO) air quality guidelines," said lead study author Yaguang Wei, an environmental health researcher at the Harvard T.H. Chan School of Public Health in Boston.

"PM2.5 is composed of tiny solids and liquids floating in the air and once inhaled, these particles can pass through the respiratory system, sneak into the blood and circulatory system, and cause serious health problems," Wei said by email.

"The most consistent and dangerous health effects identified have been cardiovascular and respiratory diseases, which are the leading causes of hospitalization, emergency room visit, and even death," Wei added.

Under WHO guidelines issued in 2005, people shouldn't be exposed to average PM 2.5 levels over 24 hours that exceed 25 micrograms per cubic meter of air (ug/m3).

Short-term exposure to fine particulate matter was associated with an increased risk of several common causes of hospital admissions including sepsis or septicemia, a life-threatening reaction to a bacterial infection in the bloodstream; fluid and electrolyte disorders; kidney failure; and intestinal obstructions. These diseases have rarely been studied in the context of PM 2.5 and hospitalizations, the study team writes in The BMJ.

For these rarely-studied diseases, each 1 ug/m3 increase in short-term average fine particulate matter levels was associated with an average annual increase of 2,050 hospital admissions, 12,216 total days in the hospital and $31 million in hospital and post-acute care costs.

Air pollution levels below safety standards set by WHO were also associated with an increased risk of hospitalizations for conditions like cardiovascular and respiratory disorders that have previously been tied to PM 2.5.

For these diseases previously linked to air pollution, each 1 ug/m3 increase in short-term average fine particulate matter levels was associated with an average annual increase of 3,642 hospital admissions, 20,098 total days in the hospital and $69 million in hospital and post-acute care costs.

Costs attributable to short-term air pollution exposure are likely far higher, said study co-author Francesca Dominici, also a public health researcher at Harvard.

"The major limitation of this study was that costs incurred after discharge, such as drug costs, readmission costs and outpatient costs could not be fully captured," Dominici said by email.

People may not be able to avoid exposure to air pollution, but they can still take some precautions, said Matthew Loxham of the University Hospital Southampton in the UK.

"All people, but especially those who have underlying health conditions which may be exacerbated by air pollution, such as heart/cardiovascular conditions, asthma, COPD, should look for local air quality levels and associated guidance, which may suggest closing windows, or avoiding strenuous outdoor exercise," Loxham, who co-authored an editorial accompanying the study, said by email.

"Furthermore, both patients and medical practitioners should be aware of the impact poor air quality can have on exacerbation of disease, to better understand and perhaps to better treat such flare-ups," Loxham said.