There is a clear, statistically significant, link between ozone levels and admissions to hospital for respiratory illnesses. This is the conclusion drawn by US researchers who performed a meta-analysis of nearly 100 studies from around the world.

Meng Ji and her colleagues from Yale University and Rice University conducted a meta-analysis of 96 studies from between 1990 and 2008 covering short-term ozone exposure and respiratory hospitalizations.

They found that, despite the fact that some of the individual studies did not find a link, their meta-analysis did find a clear link between ozone exposure and respiratory hospitalization.

Yale's Michelle Bell, co-author of the study, told environmentalresearchweb: "We had several aims with our study. We wanted to assess the overall state of scientific evidence on ozone and respiratory hospital admissions and explore variation in effect estimates, such as by study location or age group. We also wanted to investigate some of the challenges inherent in meta-analysis such as publication bias, which is caused by the tendency for authors to submit, or journals to publish, statistically significant results as opposed to null or uncertain results."

Of the 96 studies analysed, 86 studies presented only estimates for an individual city; 8 studies reported only combined estimates across multiple cities; and 2 studies gave both estimates for an individual city and combined results across cities. Although not all analyses exhibited evidence of publication bias, adjustment for publication bias generally lowered overall estimates. Emergency hospitalizations for total respiratory disease increased 4.47% per 10 ppb 24-hour ozone among the elderly without adjustment for publication bias and 2.97% with adjustment.

The researchers investigated how estimates differed by age; ozone metric; season; lag; region; disease category and hospitalization type; and they were surprised by some of the results. "It was often challenging comparing studies because different disease categories were identified through numerous International Classification of Disease (ICD) codes," said Bell. "For example, asthma and pneumonia were each identified through more than three different codes."

Comparability across studies was also hindered by other factors, such as location or age. For example, in the studies that Bell and her colleagues analysed they found that ozone was associated with increased respiratory hospitalizations among older people in the Dutch city of Rotterdam but not Amsterdam; and was associated with increased chronic obstructive pulmonary disease and pneumonia admissions in Minneapolis-St Paul, US, but significant associations were not observed in Birmingham, US.

They also found that there were large regions of the world where there was insufficient data. "We were not able to perform a regional analysis because the data are not available," said Bell. "There are large parts of Asia and Africa where the link between ozone and respiratory illness has not been investigated and there is an urgent need for studies in these parts of the world. This is particularly critical because it is these regions that are seeing a high growth in ground transportation, a major contributor to ozone levels."

The researchers published their research in Environmental Research Letters (ERL).