Bladder cancer among hairdressers: a meta-analysis

Abstract

Background: Occupational risks for bladder cancer in hairdressers by using hair products have been examined in many epidemiological studies. But owing to small sample sizes of the studies and the resulting lack of statistical power, the results of these studies have been inconsistent and significant associations have rarely been found.

Methods: We conducted a meta-analysis to determine summary risk ratios (SRRs) for the risk of bladder cancer among hairdressers. Studies were identified by a MEDLINE, EMBASE, CENTRAL search and by the reference lists of articles/relevant reviews. Statistical tests for publication bias and for heterogeneity as well as sensitivity analysis were applied. In addition, the study quality and the risk of bias were assessed using six criteria.

Results: 42 studies were included and statistically significantly increased risks around 1.3-1.7 were found for all but one analysis. The SRR increased with duration of employment from 1.30 (95% CI 1.15 to 1.48) for 'ever registered as hairdresser' to 1.70 (95% CI 1.01 to 2.88) for 'job held > or = 10 years'. No difference was found between the risk for smoking-adjusted data (SRR 1.35, 95% CI 1.13 to 1.61) and no adjustment (SRR 1.33, 95% CI 1.18 to 1.50). Studies assessed as being of high quality (n=11) and of moderate quality (n=31) showed similar SRRs. There was no evidence of publication bias or heterogeneity in all analyses.

Conclusion: In summary, our results showed an increased and statistically significant risk for bladder cancer among hairdressers, in particular for hairdressers in jobs held > or = 10 years. Residual confounding by smoking cannot be totally ruled out. Because of the long latency times of bladder cancer it remains an open question whether hairdressers working prior to 1980 and after 1980, when some aromatic amines were banned as hair dye ingredients, have the same risk for bladder cancer.

Figure 1

Flow diagram of the process of identifying and including studies. *When the study population was described in more than one study, only the study with the greatest number of cases was included. All others were excluded as duplicates.

Figure 2

Forest plot showing the influence of excluding each individual study on the summary risk ratio (SRR) obtained using all studies for bladder cancer among hairdressers. Vertical dashed line=SRR obtained using all studies, vertical solid lines=95% CI of the SRR using all studies, block with line=SRR with 95% CI obtained by omitting the mentioned study. ^†For convenience reasons only the first author is given.

Figure 3

Forest plot of studies by study design. Block=risk estimates (size displays weight), line=95% CI. ^†For convenience reasons only the first author is given.

Figure 4

Forest plot of studies by duration of job held as a hairdresser. Block=risk estimates (size displays weight), line=95% CI. ^†For convenience reasons only the first author is given. Studies with no definition or unsuitable definition for job held were excluded form this analysis.