No need to cry over spilt milk

“Wheeze 'link' to baby milk powder”, reads the headline on the BBC News website today. The site reports that a study of 170 workers in a milk powder factory in Thailand has found that extended periods of exposure to the powder “increases the risk of breathing problems, including wheezing and breathlessness”. It goes on to say that mothers and babies are safe because they have low levels of exposure to milk powder, a sentiment that is reinforced by Leanne Male, assistant director of research at Asthma UK.

This was a cross-sectional study, meaning that it only assessed people at one point in time. Therefore, it’s not possible to tell whether the workers’ respiratory problems developed before or after they were exposed to milk powder. As such, it is not possible to come to a firm conclusion that their symptoms were caused by milk powder exposure. It is important for mothers to note that although the milk powder levels in the air of the factory were described as “relatively low”, these levels are still likely to be much higher than would be expected in the home. This study should not cause mothers to be alarmed or stop using milk powder.

Where did the story come from?

Pornpen Sripaiboonkij and colleagues from the Universities of Birmingham and Oulu in Finland, and Mahdol University in Thailand carried out the research. The study was funded by the Royal Thai government. The study was published in the peer-reviewed medical journal: European Respiratory Journal.

What kind of scientific study was this?

This was a cross-sectional study of workers at a baby milk powder factory in Thailand. It compared the lung function and respiratory problems of factory workers who were exposed to the powder, to workers who weren’t.

Between September 2006 and January 2007, the researchers asked 245 workers at the factory to participate in the study. A total of 167 factory workers and 24 ‘office workers’ agreed to participate. The researchers also recruited a further 52 office workers (defined as managers, admin staff, security staff, and chauffeurs) from three other factories that made microfibre, wood furniture, and tiles. Of the factory workers, 130 were involved in the packing and production of the milk, while 22 added the vitamin mixture and 15 worked in quality control.

Each volunteer was interviewed and asked about their respiratory health and other symptoms in the past 12 months; if they had ever been diagnosed with asthma; their exposure to milk powder and other substances at their current and previous jobs, and about their lifestyles. Volunteers were also asked to perform spirometry tests that assessed the maximum amount and speed with which they could blow air out of their lungs. The researchers also obtained measurements of the dust levels in the areas of the milk powder factory where the participants worked, to see what their exposures were like. They then compared lung function and respiratory problems in the different groups of workers – factory workers or office workers.

The researchers also took into account the participants’ gender, age, education, parents’ asthma or allergies, smoking status, exposure to second-hand smoke, and work stress.

What were the results of the study?

The researchers found that the odds of experiencing wheezing or breathlessness was about two to three times higher in factory workers than in office workers. However, once factors that might be affecting the results were taken into account, the increase was no longer statistically significant. There was no significant difference in the groups in the risk of asthma. Factory workers had lower lung function than expected for their age and height.

What interpretations did the researchers draw from these results?

The researchers concluded that workers who are exposed to milk powder “even in relatively low air concentrations” have an increased risk of nasal symptoms, breathlessness and wheezing, and have reduced lung function.

What does the NHS Knowledge Service make of this study?

This study was relatively small, and its cross-sectional design means that it only assessed people at one time point. As such, it is not possible to tell whether workers’ respiratory problems developed before or after their exposure to the milk powder. Because of this, no firm conclusions can be made that their symptoms were caused by milk powder exposure. Other important points to note are:

• The factory in Thailand where this study was carried out is described as having an ‘emphasis on good hygienic conditions’, with extractor fans and some enclosed areas. However, how these conditions compared to those in similar factories in the UK and other countries, which have different health and safety regulations, is not clear.
• In this type of study, the participants are not assigned randomly to their groups; therefore, imbalances between the groups may affect results. In this case, factory workers were more likely to be male, to smoke, to be younger and have poorer education than the office workers. Once the researchers took these factors into account in their analyses, the differences between factory workers and office workers were no longer significant.
• People self-reported their symptoms and these results were not confirmed by checking doctors’ records or medical examination.
• Although the concentrations of milk powder in the factory air are described by the paper as ‘relatively low’, these levels are still likely to be much higher than would be expected in domestic situations (that is, in homes that use milk powder).

This study should not cause mothers to be alarmed or stop using milk powder.