Our results show that among hairdressers, intensity of exposure, measured by the daily number of specific tasks, and body mass index are risk factors of OA. Also, vitamin A and D intakes are greater among cases than among controls in this group. On the other hand, nutritional patterns showed no association with OA among bakers and pastry-makers. While the well known role of atopy on the occurrence of OA is confirmed in this sector, it is also suggested among hairdressers.
In our study, “work intensity” is positively associated with OA incidence among hairdressers and reflects the number and variety of techniques (discoloration, colouring, perms) performed in a typical day. Akpinar-Elci and coworkers also found a higher risk of OA in high work intensity hairdressers [19
], in accord with another study that showed an increased risk of OA, thought not significant, among hairdressers often performing hair bleaching treatments or using hair spray, compared with more infrequent users [20
]. We found no such association among bakers and pastry-makers. For Brisman et al.
, the risk of asthma or rhinitis is less dependent on the cumulative dose of inhaled flour dust than on current exposure [21
]; also, a longitudinal study showed significant associations between dust concentrations at onset of disease and the risk of asthma and rhinitis [22
]. Discrepancies in the job dropout process due to OA symptoms could in part explain these differences in the time sequence between exposure and disease incidence across occupational sectors, since bakers have more opportunities for job reclassification than hairdressers, in particular by switching to pastry-making, an activity where exposure levels are lower [23
], or to industrial bread processing rather than in the small, solo-bakeries that composed the vast majority of our study settings.
BMI was positively associated with OA among our study hairdressers (unit OR
1.24 [1.03 – 1.48]). While data on OA are scarce, many studies explored the possible link between BMI and non occupational asthma or airway inflammation [24
], showing a positive association where weight gain occurs before the onset of asthma and respiratory symptoms. This relationship between BMI and asthma could depend on gender, with a greater risk among women [32
]. This might explain why we found an association between BMI and asthma in hairdressers, a predominantly female population, and not among bakers/pastry-makers, a predominantly male population.
Atopy is a well known risk factor of work-related sensitization, especially for high molecular weight agents. The odds ratio among bakers/pastry-makers is high (10.07 [2.76 - 36.65]). Among hairdressers, the association is more uncertain, possibly due to small numbers (OR
4.94 [0.66 - 36.75]), but the role of atopy is still controversial in the literature [34
In occupational settings, nutritional factors could significantly modify host responses to environmental toxicants. According to Romieu and coll.
, “an adequate diet may inhibit, arrest, or even reverse the chain of events in toxicity, while a deficient diet could increase persons’ susceptibility to adverse environmental exposures, such as occupational allergens” [35
To the best of our knowledge, this is the first study that explored the association between nutrient intakes, vitamins in particular, and OA. There are controversies about the relation between diet antioxidant intake and asthma [5
]. Some authors assert that the increase of asthma is a consequence of a decline in antioxidant intake associated with the transition from a traditional to the modern diet [36
]. An experimental study conducted by Gu found that supplementation with antioxidant vitamins in toluene diisocyanate-treated animals, mimicking an occupational exposure, ameliorated the respiratory eosinophilia [7
]. In contrast, our results are consistent with the hypothesis that the increase in asthma and allergic diseases is favoured by an enhanced antioxidant intake associated with the greater availability of functional and antioxidant-enriched foods that might switch the balance of Th1 and Th2 towards a Th2 response [37
Our findings suggest that the role of nutritional factors might depend on the exposure context. While exposure to flour dust and atopy, but not diet, are the key predictors of OA among bakers and pastry makers, nutritional factors are associated with OA among hairdressers, subjects who encounter several pro-oxidant chemicals agents in the work place. In a normal lung, there is a balance between the toxicity of oxidants (generated through normal cellular function or exposure to an oxygen-rich environment) and the protective activities of several intracellular and extracellular antioxidant defense systems [38
]. Oxidative stress occurs when there is an imbalance between the antioxidant defense system of the body and oxidant insults, such as cigarette smoke, air pollution and infections [35
]. Chemicals agents may cause disequilibrium in this balance, through an increase in oxidant stress and a compromise of antioxidant resources, and this might result in pathophysiologic events in the lung that culminate in cellular death and pulmonary dysfunction [35
Concerning vitamin D, results are also contradictory in the literature: a first hypothesis proposed that the increase in allergy and asthma is a consequence of widespread vitamin D insufficiency [39
]. A second one, more in line with our results, links this increase to a widespread early life vitamin D supplementation for rickets prophylaxis in developed countries [39
]; now, high dose in vitro vitamin D supplementation was shown to promote Th2 differentiation [40
]. Another study also showed that vitamin D is associated with a dose-dependent reduction in transcription of Th1 cytokines, and increased expression of the Th2 cytokines [41
]. Studies on molecular epigenetic mechanisms of dietary vitamin D in lung cellular function (senescence, apoptosis, autophagia, proliferation, phagocytosis) are still ongoing. Such data might shed light on how dietary vitamin D supplementation might interact with environmental agents and give place to chronic lung diseases like asthma [42
]. Prospective studies of supplementation therapy or sub-optimal nutrient intake are needed to confirm these hypotheses.
Strengths and weaknesses of the study
The main limitation is the small number of cases which affects the statistical power of our study, possibly explaining the unstable (non significant) association between atopy and OA among hairdressers where absence of biomarkers of sensitization (specific IgE) precluded a clear assessment of asthma-like symptoms in relation to work. As in all case–control studies with other than incident cases, one should be cautious about the measures of associations since the information is collected some time after declaration of the disease. Although nested in a cohort, cases had started their symptoms with varying anteriority, due to the retrospective nature of the cohort. Thus, assessment of some risk factors might be affected by potential changes in behaviours or practices following onset of the disease (e.g. regarding the smoking status, the occupational tasks that are performed or dietary habits).
Strengths of this study lie in its design, with cases and controls coming from the same retrospective cohort study, in the accurate exposure duration history and in the diagnostic criteria we used for OA ascertainment. An interesting aspect of our study stems from the comparison of bakers/pastry-makers and hairdressers since the physical and chemical nature of the agents involved in the job processes and possibly the underlying mechanisms of asthma development differ substantially. Moreover, for the first time, nutritional factors were assessed as potential risk factors for OA. Food Frequencies Questionnaires are considered to be reproducible and to provide a useful scale for categorizing individuals according to their intake of energy and nutrients over one year [18
]. Although respiratory symptoms had begun up to 7
years prior to the medical visits, food habits are not likely to have changed so that the data provided by the questionnaires is a good proxy for nutrient intake at the time OA subjects became cases.