Evaluation of the Potential Risk of Occupational Asthma in Dentists Exposed to Contaminated Dental Unit Waterlines

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Most of the organisms isolated from dental unit waterlines (DUWL) are Gram-negative bacteria, which contain cell wall endotoxin. A consequence of endotoxin exposure is the exacerbation of asthma. This study examined the prevalence and onset of asthma among dentists and determined whether or not these were associated with the microbiological quality of DUWL in their practices. 266 randomly selected dentists (100 from rural Northern Ireland, 166 from London) completed a health questionnaire, which included questions on prevalence and time of onset of asthma. Water samples taken from the dental handpieces and surgery washbasin cold taps in all the practices were analysed using standard techniques. The questionnaire data were evaluated using both single and multivariable logistic regression. The variables considered were: smoking; surgery location; time treating patients per week; DUWL counts of Pseudomonas aeruginosa, total Pseudomonas spp., fungi, Mycobacterium spp., total aerobic colony counts (ACC) at 22 degrees C and 37 degrees C. There was no significant association between any of the variables tested in dentists and a history of asthma. A subgroup analysis was performed on dentists (n=33) who reported developing asthma since they started dental training. The final multivariable model indicated that passive smoking (OR 0.08, 95% CI 0.01-0.87, P=0.038) and total aerobic counts of >200 cfu/ml at 37 degrees C (OR 6.72, 95% CI 1.15-39.24, P=0.034) were significant variables for developing asthma since starting training as a dentist. ACC were significantly higher in London (P<0.0001) and London dentists were more likely to have developed asthma since they started training than their Northern Ireland counterparts (OR 4.4, 95% CI 1.09-17.72, P=0.033). This study suggests that the temporal onset of asthma may be associated with occupational exposure to contaminated DUWL among dentists in London and Northern Ireland.