Key Point

Smoking is a cause of asthma in adulthood; the risk persists even after quitting and appears to be greater in women than in men.

HELSINSKI—The prevalence of asthma in developed countries has been increasing, but the reasons for this increase are not clear. Lifestyle, environment, and improvements in diagnosis may all be responsible. Previous studies of smoking and asthma risk have been inconclusive. Recently, a group of Finnish researchers conducted a population-based, case-control study to determine the effects of current and past smoking on the incidence of asthma in an adult population. They concluded that smoking is a significant cause of asthma in adulthood.¹

“Previously, it has been controversial whether smoking causes asthma or only causes COPD,” said Maritta S. Jaakkola, MD, DSc, Senior Clinical Lecturer in Respiratory and Occupational Medicine at the University of Birmingham in the United Kingdom. “We were able to show that smoking causes asthma. This means that reducing smoking can prevent a considerable portion of adult-onset asthma cases.”

THE STUDY

The patient population was taken from adults between 21 and 63 who lived in the Pirkanmaa Hospital district in southern Finland. All newly diagnosed cases of asthma were recruited during a period of two and a half years. Diagnostic criteria for asthma were the presence of at least one asthma symptom and airway obstruction that was reversible with a bronchodilator. A total of 521 patients were recruited for the study, along with 932 controls who were matched for age and selected randomly from among residents of the study area.

Information on smoking was collected via self-administered questionnaires that inquired about current smoking status, number of cigarettes smoked per day, and time since starting smoking or since quitting. Lung function data, including spirometry, two-week diurnal peak expiratory flow surveillance, and a two-week oral corticosteroid test (if necessary for diagnosis) were obtained for those suspected of having asthma.

ASTHMA RELATED TO SMOKING

Asthma patients were younger, more likely to be women, had more relatives with allergies, and fewer years of education than controls. They were also more likely than controls to be current or former smokers.

Asthma risk was strongly related to both former and current smoking, with ex-smokers having the greater risk. In addition, asthma risk increased with number of cigarettes smoked per day up to 14 but was lower in those who smoked 15 or more cigarettes per day. The same trend held true for cigarette years, with asthma risk being higher in the 1-to-199 cigarette years category, and lower in the over 200 category.

In never-smokers, women had a 57% increased risk of asthma compared with their male counterparts. Male ex-smokers had a slightly increased risk of asthma, but women who were either current or ex-smokers had significant increases in asthma risk. In fact, the joint effect of female sex and current smoking added up to a 143% increase in asthma risk, and female ex-smokers had a 138% increased risk.

A PARADOXICAL EFFECT AMONG HEAVY SMOKERS

Asked why asthma risk was lower in the heaviest smokers, Dr. Jaakkola postulated that these individuals are part of a subgroup that does not experience adverse effects from smoking. Generally, people who have adverse effects from cigarette smoke either stop smoking or reduce the daily number of cigarettes they smoke. The fact that some individuals are able to smoke heavily without developing asthma points to a genetic contribution or other factors not studied.

How does smoking cause asthma? “Tobacco smoke contains several irritants that have been shown to cause chronic inflammation in the airways and in the alveoli. Such chronic inflammation can lead to asthma,” Dr. Jaakkola explained. “In addition, smoking has been shown to modulate the immune system and to impair normal repair systems leading, potentially, to asthma.” She added that in both animals and humans, hypersensitivity to allergens is enhanced in the presence of tobacco smoke.

The investigators pointed out that—unlike previous studies—their study controlled for potential confounders such as pets, dampness and mold problems, and environmental tobacco smoke, thus strengthening the evidence for an association between smoking and asthma. “The strength of the current study,” they wrote, “lies in the accuracy of the assessment of both exposure and asthma.”

One question to pursue in future studies, suggested Dr. Jaakkola, “is how long the risk of asthma is increased after a person quits smoking. Another would be why women are more susceptible to the harmful effects of tobacco smoke.”

—Gale Jurasek

Reference
1. Piipari R, Jaakkola JJK, Jaakkola N, Jaakkola MS. Smoking and asthma in adults. Eur Respir J. 2004;24:734-739.

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