Why does respiratory surveillance fail to detect new onset occupational asthma?

At first glance this paper finds that surveillance only detected 11/60 new cases cases of occupational asthma in workers exposed to sensitising agents in Finland, and also states that no cases were detected by spirometry who were asymptomatic. However we are also told that the median time from first symptom to diagnosis was 2.1 years is those detected by surveillance and 2.2 years in those detected (?by their occupational health services) outside formal surveillance visits, and that spirometry was abnormal in 8/15 in whom it was performed at the time investigation was initiated. An alternative explanation would be that symptoms of asthma or rhinitis, or abnormal spirometry, identified at surveillance, did not trigger an evaluation for occupational asthma, or that surveillance was being carried out too infrequently (we have no information on this).

References

	Suojalehto H, Karvala K, Haramo J, Korhonen M, Saarinen M, Lindström I, Medical surveillance for occupational asthma—how are cases detected? , Occup Med, 2017;67:159-162,doi.org/10.1093/occmed/kqw101

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