Asthma rates decline due to less cigarette smoke?

There was an interesting headline in the Globe and Mail on Wednesday; it read: “Asthma rates decline among children; Statscan cites less cigarette smoke “

The article, by Jill Mahoney, claims that: “So few parents now smoke that there were not statistically significant differences in asthma rates between kids in smoking and non-smoking households between 2006-07 and 2008-09,” before going on to quote from the StatCan article: "This suggests that . . . adult smoking rates have become low enough that parental smoking has ceased to be [a] major cause of asthma in young children." That's quite a leap in logic.

But, there are a few problems with this “suggestion”. For example, according to the Asthma Society of Canada, the causes of asthma remain unknown. “Asthma is not contagious. While its causes are still unknown, researchers have determined that asthma can be caused by both hereditary (inherited) and environmental factors.”

So, in fact, parental smoking has never been identified as a “major” cause of asthma. Even the last US Surgeon General's report stated that evidence implicating secondhand smoke as a cause of asthma is merely suggestive.

And, the truncated quote from the StatCan article omits the claim by study author, Eleanor Thomas, that “reduced exposure to cigarette smoke contributed to declines in asthma over time.”

The full quote from the Statcan article reads: “Again, this suggests that reduced exposure to cigarette smoke contributed to declines in asthma over time, and that adult smoking rates have become low enough that parental smoking has ceased to be major cause of asthma in young children.”

That's two misleading claims based on that single quote, because Thomas is saying simply that the evidence suggests SHS exposure may have contributed to the alleged decline in asthma rates. But to do so, she ignores decades of previous data.

Physicians for a Smoke-free Canada has a chart showing that smoking prevalence began to decline in 1965 for men and about ten years later for women. Smoking prevalence rates have continued to fall for both men and women since 1975.

A “ fact sheet” from the Asthma Society notes that there was a dramatic increase in the incidence of asthma, beginning in 1980, which didn't level off until sometime after 2000. The Globe and Mail article points out this increase: “Researchers in western countries have generally found that asthma rates among children increased steadily for several decades before levelling off or even declining.”

The Asthma Society fact sheet also notes a dramatic increase in asthma rates during this period: “The prevalence of asthma among adults (15 years of age and over) has been increasing over the last 20 years. 1979 asthma incidence was 2.3 per cent, in 1988 it was 4.9 per cent, and by 1994 it had increased to 6.1 per cent of the population. “

Just to make it clear, asthma rates had been steadily increasing at the same time that smoking prevalence had been falling. And, declining rates of smoking prevalence, coupled with smoking bans in public places, including public transportation, should have resulted in reduced exposure to SHS beginning around 1975. Shouldn't we have seen at least some slight decline in asthma rates during that time frame, rather than such a sizable increase (roughly 165%).

Yet we have a claim by the study author that her study “suggests” that reduced exposure to cigarette smoke contributed to declines in asthma over time.

The real question is why the incidence of asthma went up in the first place. Smoking can't be tagged as the cause of the phenomenal increase in asthma rates between 1979 and 2000/2001; smoking prevalence was declining during that same period.

And, claiming that Thomas was only considering asthma rates for children ages 2 to 7 won't justify the claim that reduced exposure to SHS contributed to the recent decline. As noted in her article, “The prevalence of asthma among children aged 2 to 7 rose steadily until 2000/2001 and then fell.” Asthma rates were increasing across the board, in all age groups.

The Globe and Mail article includes another quote from the study: "A wide range of environmental factors, including reduced exposure to cigarette smoke, may have contributed to these trends."

That's an accurate quote from the study summary.

However, there's another quote in the body of the StatCan article which elaborates: “A number of environmental factors may be related to the recent declines in childhood ear infections and asthma: changes in the population structure; changes in diagnostic practices; decreases in the prevalence of respiratory allergies; improvements in air quality; changes in hygiene practices (particularly, in child care settings); and reductions in children’s exposure to cigarette smoke at home. An investigation of most of these factors is beyond the scope of this paper, but the possible role of exposure to cigarette smoke can be considered.”

So, the evidence, as related by Thomas and the Globe and Mail, is only one of many potential factors which may have contributed to the decline. What should we make of this study and the blaring headline that declining rates of asthma in children are the result of reduced cigarette smoke?

No investigation was made to explain the tremendous increase in asthma rates at the same time smoking prevalence was declining. No investigation was conducted to determine the role of the numerous other factors which may have contributed to the apparent decline among children between 2 and 7 years old. They were “beyond the scope of this paper”.

No comparison was made between asthma rates pre-1980, when smoking prevalence was high and almost all kids would have been exposed to SHS. The comparison was made between rates of asthma in 2000/2001, when the increase in asthma rates had apparently peaked, and 2006/2007 when the first downward trend was noted.

It's difficult to believe that the decline in smoker prevalence from 1975 onwards had no affect on the incidence of asthma until 2006/2007, and then, only among those aged 2 to 7.

Objective science? Or a blatant attempt to justify smoking bans and smoker denormalization? You be the judge.