Individual susceptibility to noise-induced hearing loss: choosing an optimal method of retrospective classification of workers into noise-susceptible and noise-resistant groups.
OBJECTIVE
Individual susceptibility to noise-induced hearing loss (NIHL) depends on the interaction between intrinsic and environmental factors. To proceed with the study on NIHL susceptibility genes an appropriate selection of workers susceptible and resistant to noise is crucial. The aim of the study was to compare four different methods of subject classification by the susceptibility to NIHL in a group of 949 workers of an electric power plant exposed to steady-state noise at the workplace.


METHODS
One method based the classification of the workers on the international reference standard ISO 1999:1990; from the entire group of workers, 10% of the subjects with the worst hearing thresholds (HT) in the model were categorized as susceptible to NIHL, whereas 10% of the subjects with the best HT were categorized as resistant to noise. According to three other methods, the entire group of workers was first divided into subgroups by age, duration of employment and the level of noise, and then 10% of the subjects at each HT extreme were selected.


RESULTS
The first classification allowed to achieve an excellent separation between HT of the susceptible and resistant subgroups. The susceptible subgroup was significantly younger than the resistant one, showed a shorter duration of employment and a lower level of noise exposure, which is in line with the definition of increased vulnerability to NIHL. The three other methods produced poorer separation of HT with smaller or no gap between HT values in subgroups.


CONCLUSIONS
The selection of subjects from the entire worker population of a given industry based on the ISO 1999:1990 standard can be regarded as the most reliable method of classification of noise-susceptible and noise-resistant workers to be used in the future genetic studies on NIHL susceptibility genes.
Sliwińska-Kowalska
Mariola
M
Department of Audiology and Phoniatrics, Nofer Institute of Occupational Medicine, łódź, Poland. marsliw@imp.lodz.pl
Dudarewicz
Adam
A
Kotyło
Piotr
P
Zamysłowska-Szmytke
Ewa
E
Pawlaczyk-łuszczyńska
Małgorzata
M
Gajda-Szadkowska
Anna
A
D000328:Adult; D000367:Age Factors; D001301:Audiometry, Pure-Tone; D004651:Employment; D006317:Hearing Loss, Noise-Induced / Q000145:classification*; D006801:Humans; D008297:Male; D008875:Middle Aged; D009623:Noise, Occupational / Q000009:adverse effects*; D016273:Occupational Exposure / Q000331:legislation & jurisprudence*; D012016:Reference Values; D012189:Retrospective Studies; D012307:Risk Factors