Asthma is caused by inflammation of the airways. If a person has asthma, the bronchi will be inflamed and more sensitive than normal. If a person comes in contact with something that irritates their lungs, the airways will narrow, the muscles around them tighten and the production of mucus will increase. This will make it difficult to breathe, will cause wheezing and coughing and make a person’s chest feel tight. When these symptoms are really bad it is called an asthma attack or an 'acute asthma exacerbation'. Asthma attacks may require hospital treatment and can sometimes be life-threatening.
Active smoking and asthma
Evidence that active smoking increases risk of asthma
- Active smokers are 1.61 times more likely to develop asthma
The increased risk of asthma from active smoking was similar in studies conducted in Europe compared to elsewhere
What evidence was used?
The present review started in January 2013, and was designed to identify and update previous systematic reviews of studies that assessed the effect of active smoking on asthma.
In March 2013, a previous systematic review into active smoking and asthma was identified (King 2004). The methodological quality of the review was assessed using a recognised assessment tool (AMSTAR). The review scored highly in terms of: defining inclusion/exclusion criteria; performing a comprehensive literature search using MEDLINE and EMBASE (from 1994 to 2004); and providing characteristics of the included studies. However, the review scored poorly overall due to: the lack of duplicate study selection and data extraction; lack of a list of studies excluded from the review; lack of assessment of the methodological quality of the included studies; lack of combining the results of the studies using appropriate methods; lack of assessment of publication bias; and a lack of a conflict of interest statement. Additionally, only significant findings from the included studies were presented in the review. Therefore, the included studies were re-assessed by the project team in terms of data extraction and methodological assessment.
Eight studies were identified from the review which assessed the effect of active smoking on developing asthma in adults. All of the included studies were deemed to use prospective cohort study designs to determine the association between smoking and the development of asthma; however, two of the studies referred to the study designs as repeated cross-sectional surveys (Larsson 1995; Lundback 2001).
All of the included studies measured active smoking status through self-reports; however one study focused on previous smoking in non-smokers (McDonnell 1999). Asthma was ascertained through physician reports in the majority of studies; however, two studies relied on self-reports (Romieu 2003, Norman 1998).
All but two of the included studies (Troisi 1995, McDonnell 1999) were conducted in Europe.
The methodological quality of the eight studies included in the meta-analysis, as judged by the Newcastle-Ottawa scale score, gave an overall median score of 6.5 (range 4−8). Before the quality was evaluated, a score of 7 had been selected as the lowest score that was acceptable as ‘high quality’; four (50%) of the studies were judged to be high quality.
Detailed findings and data
Smokers were 1.61 times more likely to develop asthma when compared to people who had never smoked (pooled relative risk ratio 1.61, 95% confidence interval 1.07 to 2.42). Click here to see a forest plot of the findings – Figure A
A high level of heterogeneity was seen between the studies, predominately related to the low quality study by Troisi 1995, which reported a significantly reduced risk of asthma in smokers. A sensitivity analysis was performed excluding the Troisi 1995 study, which found that smokers were 1.81 times more likely to develop asthma when compared to people who never smoked (pooled relative risk ratio 1.81, 95% confidence interval 1.37 to 2.38).
To explore the impact of geographical location, a subgroup analysis was conducted based on whether the studies were conducted in Europe or elsewhere. In studies conducted in Europe, smokers were 1.77 times more likely to develop asthma when compared to those who had never smoked. Click here to see a forest plot of the findings – Figure B.
To explore the impact of the methodological quality of the eight studies on the review, a subgroup analysis was performed based on studies with higher (≥7) and lower (<7) quality scores. Click here to see a forest plot of the findings – Figure C.
The higher quality studies found more than a doubling in risk of developing asthma in smokers; however, the difference in the pooled results between the higher and lower quality studies was not statistically significant (test for subgroup differences, p=0.17).
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Eagan TML, Bakke PS, Eide GE, Gulsvik A. Incidence of asthma and respiratory symptoms by sex, age and smoking in a community study. European Respiratory Journal 2002; 19: 599-605.
Larsson L. Incidence of asthma in Swedish teenagers: relation to sex and smoking habits. Thorax 1995; 50: 260-264.
Lundbäck B, Rönmark E, Jönsson E, Larsson K, Sandström T. Incidence of physician-diagnosed asthma in adults – a real incidence or a results of increased awareness? Report from the Obstructive Lung Disease in Northern Sweden Studies. Respiratory Medicine 2001; 95: 685-692.
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Romieu I, Avenel V, Leynaert B, Kauffmann F, Clavel-Chapelon F. Body mass index, change in body silhouette, and risk of asthma in the E3N cohort study. American Journal of Epidemiology 2003; 158: 165-174.
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Troisi RJ, Speizer FE, Rosner B, Trichopoulos D, Willett WC. Cigarette smoking and incidence of chronic bronchitis and asthma in women. Chest 1995; 108: 1557-1561.
Active and passive smoking and asthma exacerbations
Evidence that active and passive smoking increases risk of asthma exacerbations
- In people with asthma, smoking increases the risk of asthma exacerbations by 1.71 times
- Pregnant women with asthma who smoke have more asthma exacerbations per year and poorer asthma control
- Children with asthma who are exposed to passive smoke have an increased risk of multiple hospital admissions for asthma compared to non-exposed children by 2.55-3.25 times
What evidence was used?
In April 2012, a systematic review was performed to identify studies which looked at the effect of active and passive smoking on exacerbations of asthma.
A total of 13 studies were found, five of which met the inclusion criteria for this review. Four of the studies used a prospective cohort design, and the remaining study used a nested case control design.
Two of the five studies assessed the effect of active smoking, with one study performed in an adult population and the other study in pregnant women. Three studies assessed the effect of passive smoking, with two studies being performed in adult populations and the other study in a population of children.
The studies identified whether the people they were investigating were active or passive smokers through self-report, and one study also measured passive smoke levels using hair samples.
One study was conducted solely in the UK, two studies were conducted solely in the USA, one study was conducted in both the USA and UK, and the final study was conducted in Turkey.
The quality of the five studies (as judged by the Newcastle-Ottawa scale score) ranged from 4 to 7, with two studies being awarded 4 (lower quality <7), and the remaining three studies being awarded a score of 7 (high quality ≥7).
Detailed findings and data
Findings from one study demonstrated that people with asthma who were current or ex-smokers had a higher risk of subsequent asthma exacerbations than people with asthma who had never smoked (odds ratio 1.71, 95% confidence interval 1.48 to 1.97) (Himes 2008).
A second study of 80 pregnant women with asthma found that ever-smoking significantly increased the number of severe asthma exacerbations; current smokers also had poorer asthma control, compared to never-smokers (Murphy 2010).
A study of 778 non-smoking adults demonstrated that exposure to passive smoke was not significantly associated with an increased risk of admission to hospital for asthma (Eisner 2005).
Another study of 164 non-smoking adults reported that living in a household with smokers significantly increased the risk of being restricted in daily activities (odds ratio 1.61, 95% confidence interval 1.06 to 2.46), but had no significant effect on increased risks of cough, shortness of breath or being awakened by asthma (Ostro 1994).
A third study of 140 children with asthma aged 3−15 years found that having a smoker in the house more than doubled their risk of multiple hospital admissions for asthma per year (odds ratio 2.55, 95% confidence interval 1.12 to 5.82). Furthermore, exposure to maternal smoking more than tripled the risk of multiple hospital admissions for asthma per year (odds ratio 3.25, 95% confidence interval 1.13 to 8.85) (Gurkan 2000).
Eisner MD, Klein J, Hammond SK, Koren G, Lactao G, Iribarren C. Directly measured second hand smoke exposure and asthma health outcomes. Thorax 2005; 60: 814-821.
Gurkan F, Ece A, Haspolat K, Derman O, Bosnak M. Predictors for multiple hospital admissions in children with asthma. Canadian Respiratory Journal 2000; 7: 163-166.
Himes BE, Kohane IS, Ramoni MF, Weiss ST. Characterization of patients who suffer asthma exacerbations using data extracted from electronic medical records. AMIA Annual Symposium Proceedings 2008: 308-312.
Murphy VE, Clifton VL, Gibson PG. The effect of cigarette smoking on asthma control during exacerbations in pregnant women. Thorax 2010; 65: 739-744.
Ostro BD, Lipsett MJ, Mann JK, WienerMB, Selner J. Indoor air pollution and asthma. Results from a panel study. American Journal of Respiratory and Critical Care Medicine 1994; 149: 1400-1406.