Epidemiology

The prevalence of hypersensitivity pneumonitis (HP) is not precisely known. It most probably varies with the antigen, the exposure concentration, and as yet unidentified host factors. One US claims study found the yearly prevalence to be 1.67 to 2.71 per 100,000 people, and the 1-year cumulative incidence to be 1.28 to 1.94 per 100,000 people.[8]​ The annual incidence is estimated to be 0.89 to 1.53 per 100,000 people in Denmark, and 1.14 to 2.16 per 100,000 people in South Korea.[9][10]​​​​ The epidemiology of HP also varies with occupation. One study using data from the UK’s occupational health reporting scheme found that the most common cause of occupational HP was metalworking fluids, followed by farming and birds.[11] It has been estimated that HP occurs in 6% to 21% of pigeon breeders. One Danish study reported that the hazard ratio for pigeon breeders developing HP is 14.36 (95% CI: 8.10 to 25.44).[12] A complicating factor in occupational causes of HP is the healthy worker effect and high turnover among those employees who develop HP; therefore, HP may be under-reported.​[13][14][15][16]​​ A report using national data from 1988 to 2016 suggested that deaths from HP increased in the US over that period.[17] A cohort study carried out at a single hospital in Denmark between 2003 and 2009 found that 7% of patients diagnosed with interstitial lung diseases had HP.[18]

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