Short term exposure to fine particulate matter in the air is related to several new causes of hospital admissions, according to a study which suggests that even pollution levels below international air quality guidelines set by WHO may be hazardous to health.
The researchers, including those from Harvard University in the US, analysed more than 95 million Medicare hospital insurance claims for adults aged 65 or older in the US from 2000 to 2012, and linked these with estimated daily exposure to PM2.5 based on data from the US Environmental Protection Agency (EPA).
The study, published in The BMJ, confirmed several previously established causes of hospital admission that are associated with short term exposure to particulate matter 30 times smaller than the width of a single human hair -- PM2.5.
It noted that exposure to PM2.5 is linked to increased risk of hospitalisation for illnesses like heart and lung diseases, diabetes, and Parkinson's disease.
The causes for hospital admission among the patients were classified into 214 mutually exclusive disease groups. They estimated the increased risk of admission, and the corresponding costs associated with a 1 microgramme per cubic metre increase in short term exposure to PM2.5 for each disease group.
The findings of the study revealed that each 1 microgramme per cubic metre increase in PM2.5 was associated with 2,050 extra hospital admissions.
The researchers said such an increase in PM2.5 was also linked to 12,216 extra days in the hospital, and USD 31 million additional healthcare costs for diseases not previously associated with PM2.5 such as sepsis, kidney failure, urinary tract and skin infections.
The study noted that these associations remained even when the analysis was restricted to the days when PM2.5 concentration was below the WHO air quality guideline, suggesting that they need updating.
The researchers said the study had a few limitations, such as being unable to fully capture costs after discharge, or in taking account of other factors that may trigger hospital admission, such as smoking, alcohol consumption, and drug use.
However, they said the study's large sample size over a 13-year period, and results that remained similar after further analyses, suggested the results were robust.
According to the researchers, the newly discovered causes of hospital admissions confirmed several already known associations, even at daily PM2.5 concentrations below the current World Health Organization (WHO) guidelines.
"Substantial economic costs were linked to a small increase in short term PM2.5," the researchers wrote in the study. They cautioned that scientists' knowledge of the health effects of tiny particulate matter in the air is still lacking in many areas.
According to the researchers, the newly associated diseases represent around a third of the total PM2.5 associated effect, suggesting that current figures for PM2.5 associated illness "might be considerable underestimates."
"Clearly, there is still much to learn, but we should not mistake knowledge gaps for paucity of evidence. The sooner we act, the sooner the world's population will reap the benefits," they said in a statement.
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