Night shift work increases risk of severe asthma in women

Women who work night shifts are more likely to suffer with moderate or severe asthma compared to women who work in the daytime, according to a study published today (Monday) in ERJ Open Research.

The research, which included more than 270,000 people, found no such link between asthma and working nightshifts in men.

The study was by Dr. Robert Maidstone from the University of Manchester, UK, and colleagues.

He said: "Asthma disproportionately affects women. Women generally have more severe asthma, and higher rate of hospitalization and death from asthma compared to men.

"In our previous research we found a higher risk of moderate or severe asthma in nightshift workers, so we wanted to see whether there were further differences between the sexes."

The researchers used data from the UK Biobank. They included a total of 274,541 working people and found that 5.3% of these had asthma, with 1.9% suffering with moderate or severe asthma (meaning they were taking an asthma preventer inhaler and at least one other asthma treatment, such as an oral steroid). They categorised these people according to whether they worked only during the day, only nightshifts, or a combination of the two.

Their analysis revealed that, overall, women who work shifts are more likely to have asthma. Women who only work nightshifts are around 50% more likely to suffer with moderate or severe asthma compared to women who only work in the daytime.

The risk of asthma in men did not alter according to whether they worked days or nights.

This is the first study to evaluate sex differences in the relationship between shift work and asthma. We found that permanent night shift-workers had higher odds of moderate-severe asthma when compared to corresponding day workers.

This type of research cannot explain why shift work and asthma are linked; however, it could be because shift work disrupts the body clock, including the levels of male and female sex hormones. High testosterone has previously been shown to be protective against asthma, and so lower testosterone in women could play a role. Alternatively, men and women work different types of shift jobs, and this could be a factor."

Dr. Robert Maidstone, University of Manchester

In postmenopausal women, the risk of moderate or severe asthma was almost doubled in night workers, compared to day workers, in those not taking hormone replacement therapy (HRT).

Dr. Maidstone added: "Our results suggest that HRT might be protective against asthma for nightshift workers, however further research is needed to test this hypothesis in prospective studies and randomized controlled trials."

The researchers plan to study whether sex hormones play a role in the relationship between shift work and asthma by using data from the UK Biobank and from Our Future Health, a new health research programme in the UK population.

Professor Florence Schleich from the European Respiratory Society's expert group on airway diseases, asthma, COPD and chronic cough, based at the University of Liège, Belgium, and was not involved in the research. She said: "Asthma is a common, long- term condition that affects millions of people worldwide. We know that women are more likely to have asthma, to have worse asthma and more likely to die from asthma, but we do not fully understand why.

"This research suggests that working nightshifts could be a risk factor for asthma in women, but not in men. The majority of workers will not have an easy option of switching their shift pattern, so we need further research to verify and understand this link and find out what could be done to reduce the risk for women who work shifts."