Behind the Headlines

Tuesday July 21 2009

Traffic pollution and smoking mothers were linked with asthma in children

“Stressed-out mums and dads could be to blame for the epidemic in childhood asthma,” the Daily Mirror reported. It said a study of almost 2,500 healthy children over three years found that those exposed to more air pollution were more likely to develop asthma, and that parental stress increased this risk even more.

This study suggests that socioeconomic status and parental stress may increase the risks associated with traffic-related pollution and maternal smoking on pregnancy although by themselves they do not appear to affect asthma risk.

The study does have some limitations, including the fact that socioeconomic status was based only on the level of parental education, parental stress was only measured at one time point and that many of the factors assessed including children’s diagnoses, were based on reports from the parents only and   not independently verified. Further studies will need to confirm these results.

 

Where did the story come from?

The research was carried out by Dr Ketan Shankardass and colleagues from the Li Ka Shing Knowledge Institute of St Michael’s Hospital, Canada and two US universities. It was funded by the National Institute of Environmental Health Sciences, the US Environmental Protection, a National Cancer Institute Grant, the Hastings Foundation, and the Canadian Institutes of Health Research.

The study was published in the peer-reviewed scientific journal, Proceedings of the National Academy of Sciences of the United States of America.

 

What kind of scientific study was this?

This research is part of a prospective cohort study called the Southern Californian Children’s Health Study. The study has already shown that traffic-related pollution and maternal smoking during pregnancy were associated with an increased risk of asthma in children. The current analyses aimed to look at whether socioeconomic status and parental stress might increase the risk of asthma even further in children exposed to pollution and maternal smoking.

The researchers enrolled children between five and nine years of age, who were attending schools in 13 communities in southern California in 2002 and 2003. All students from the study communities were asked to participate and 65% (5,349 children) returned the study questionnaires given. These questionnaires asked about the children’s health, including questions about chest-related conditions and allergies. They also asked about other factors such as race, gender, where they lived, type of medical insurance coverage, whether they had been exposed to maternal smoking during pregnancy or if anyone in the home currently smoked on a daily basis, and family history of asthma. Each household’s exposure to traffic-related pollution was estimate, based on their location and distance from local ‘measured’ sources of pollutants, such as traffic.

Parental education was used as a measure of the family’s socioeconomic status. Parental stress levels were measured at the start of the study using a standard question (the Parental Stress Scale, PSS):

“In the last month, how often have you felt:

  • (a) that you were unable to control the important things in your life;
  • (b) confident about your ability to handle your personal problems;
  • (c) that things were going your way; and
  • (d) your difficulties were piling up so high that you could not overcome them.”

The parents rated how often they felt each of these on a scale of zero to four, and the scores were summed to give a score from zero to 16 (higher scores indicating greater stress).

The children’s living conditions were assessed using questions about: type of home; whether the child lived elsewhere for more than 50 days a year; previous water damage or flooding in the home; the presence of mould or mildew on household surfaces, the presence of a musty smell and cockroaches or pets in the home. There were also questions about whether gas stoves, air conditioners, humidifiers or vaporisers were used in the home and whether the child’s bedroom had a carpet.

The researchers excluded children who had already been diagnosed with asthma those who had wheezing episodes, and those who did answer or know. Children whose exposure to traffic-related pollution could not be assessed, and those who were lost to the follow up carried out in the first year of the study were also excluded. This left 2,497 children for analysis. Just over half of the children (55%) were Hispanic, just over a third non-Hispanic white (36%), 3% African American, and 6% of other races or ethnic groups.

During the three years of follow up, the children’s parents filled out annual questionnaires about the children’s health, including whether they had been diagnosed with asthma. The researchers looked at how the factors assessed affected a child’s risk of developing asthma during the follow up. In particular, they looked at the effects of traffic-related pollution and maternal smoking during pregnancy, and whether parental stress or socioeconomic stress affected the level of risk associated with these factors. They took into account the factors that they measured that could affect outcome in their analyses, such as age, gender, and ethnicity.

 

What were the results of the study?

About 21% of families were defined as being of low socioeconomic status because the parents had not finished high school. On average, parental stress score was 3.85 on the PSS (the most stressed score possible was 16).

Just under 5% of children (120 children) developed asthma during the follow up. African American children were more likely than Hispanic children to develop asthma. Children who were underweight, had a history of chest illness or allergy, who lived in a home with a musty smell or whose parents had asthma were also more likely to develop asthma.

Low socioeconomic status and higher parental stress alone did not increase the risk of developing asthma.

Increasing exposure to traffic-related pollution did increase the risk of childhood asthma. Among children in low socioeconomic status families or those with parents with higher levels of stress, the effects of traffic-related pollution were greater than in children from high socioeconomic status families or parents with lower levels of stress. The effects of socioeconomic status were reduced if parental stress was taken into account.

Exposure to maternal smoking during pregnancy did not significantly increase the risk of childhood asthma overall. However, exposure to maternal smoking during pregnancy did significantly increase risk of childhood asthma in children from low socioeconomic status families or those with parents with higher levels of stress.

 

What interpretations did the researchers draw from these results?

The researchers conclude that, “children whose parents perceived their lives as unpredictable, uncontrollable, or overwhelming had increased risk of new onset asthma associated with [traffic-related pollution] and maternal smoking during pregnancy”. They also say, “understanding the role of air pollution in the causation of complex diseases like asthma requires consideration of how social factors may modify the effects of environmental exposures”.

 

What does the NHS Knowledge Service make of this study?

This study suggests that socioeconomic status and parental stress may exacerbate the risks associated with traffic related pollution and maternal smoking on pregnancy, although by themselves they do not appear to affect asthma risk. It has the advantage of having collected the data over time (prospectively), however, there are some limitations to note:

  • As with all studies of this type, the relationship between parental stress, socioeconomic status and asthma risk may be due to other factors that are not balanced between the groups. The researchers did try to take some of these factors into account in their analyses, but this may not have fully removed their effects and could not remove the effects of unknown or unmeasured factors. In particular, parental stress may be an indicator of other problems that could affect a child’s risk of asthma.
  • Socioeconomic status was defined based only on the level of parental education. A more accurate measure may have been attained if the researchers had also looked at other factors such as family income and area of residence. In addition, a large proportion of Hispanics were of low socioeconomic status (35.0%) compared to those with non-Hispanic background (4.0%) and although ethnicity was taken into account in the analysis, it could still have affected the results.
  • Only a small number of mothers (6.3%) were reported to smoke during pregnancy. The results for this group would be more reliable if a larger number of exposed children had been available for analysis.
  • Many of the factors assessed were based on the parents’ reports, including children’s asthma diagnoses and maternal smoking during pregnancy. This may mean that some of the information may not have been accurate, due to missed diagnoses, inaccurate recall or misreported information.
  • Parental stress was only measured at one time point (the month preceding the start of the study), and this may not have accurately reflected their usual stress levels or stress levels over a longer period.
  • The results may not apply to children from age groups or ethnic groups different to those studied.

Further studies will be needed to confirm these results.

Analysis by Bazian

Edited by NHS Choices

Links to the headlines

Stressed parents up asthma risk. BBC News July 21 2009

Childhood asthma 'caused by stressed parents'. The Daily Telegraph, July 20 2009

Stress link to asthma. Daily Mirror, July 21 2009

Links to the science

Ketan Shankardass K, McConnell R, Jerrett M, et alParental stress increases the effect of traffic-related air pollution on childhood asthma incidence. PNAS 2009; Published online before print July 09

Further reading

Yorke J, Fleming SL, Shuldham C. Psychological interventions for children with asthma. Cochrane Database of  Systematic Reviews 2005, Issue 4

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