Introduction Epidemiological studies indicate an increased asthma prevalence among cleaning professionals compared to other jobs.

Objectives In a multi-disciplinary project in Denmark on spray cleaning products we investigated the risk of asthma among professional cleaners in a nationwide population-based register study.

Methods In a register-based matched cohort study, 16–50 year-old professional cleaners were identified according to yearly assigned job and industrial codes for cleaning. The references was workers with other manual jobs/service workers. Asthma was defined from national registers on hospitalisation and prescribed asthma medication (person years: cleaners = 1,014,893; references = 2,777,052). The associations between recent (previous year) and preceding cumulated cleaning years and incidences of asthma were estimated using Poisson regression analysis. The analyses were repeated in an inception cohort among workers aged 16–20 years at start of follow-up (person years: cleaners = 153,549; references = 423,506).

Results The risk of asthma was not increased for recent cleaning compared to references (adjusted incidence rate ratio [aIRR]=1.02 [95% confidence interval (CI) 0.99–1.04]). Similar results were seen for recent cleaning in the inception cohort. Cumulated cleaning years (up to 10 years) showed decreased risk of asthma (aIRR = 0.74 [95% CI 0.63–0.88] for 10 compared to 1 year of cleaning). However, in the inception cohort (up to 6 years) cumulated cleaning years were associated with increased asthma risk (aIRR=2.53 [95% CI: 1.38–4.64] for 6 compared to 1 cleaning year).

Conclusion In this study, asthma risk increased with cumulated years of cleaning in the inception cohort. This indicates a strong healthy worker selection and suggests that long-term professional cleaning may be associated with increased risk of asthma. However, in the full population we could not confirm that recent work within cleaning was associated with increased risk of asthma; furthermore, cumulated years of cleaning was inversed associated with asthma.