The ammonia is released from livestock manure and urine and the overuse of synthetic nitrogen fertilisers. The gas drifts into cities and reacts with other air pollutants to form tiny particulate matter, called PM2.5, which is the deadliest form of air pollution.
These particles cause £8bn a year in health damage in the UK, the scientists calculate. Globally, 39% of PM2.5 is derived from ammonia and results in $420bn (£320bn) of health damage, according to the study, published in the journal Science.
Ammonia can be trapped on farms by sealing manure pits or injecting the waste under fields, and by the more efficient use of fertiliser. Such action saves £23 in health damage for every £1 spent in the UK, say the researchers, with the global cost benefit ratio being 4:1.
In the past, the burning of fossil fuels by vehicles and industry produced large amounts of PM2.5 but pollution controls have cut levels significantly in developed nations. However, ammonia emissions have barely fallen in the UK since 1980. This means agriculture is now responsible for a larger share of PM2.5 in the UK. Pollution from wood burning stoves has also risen in prominence.