Secondhand smoke (SHS) has been recognized as a human carcinogen since 2006; it is causally associated with ischemic heart disease and lung cancer in nonsmokers, and there is no known safe threshold for exposure.1 According to the World Health Organization, exposure to SHS causes more than 1 million deaths a year.2

Protection from SHS exposure in Spain improved with the approval of Law 28/2005 on Health Measures Against Smoking3 and its subsequent amendment through Law 42/2010.4 Both laws marked a public health milestone and positioned Spain at the forefront of international efforts to legally address the problem of involuntary exposure to tobacco smoke.

Two indicators can be used to assess SHS exposure in the general population: prevalence of exposure and burden of mortality attributable to exposure. The first estimates of SHS exposure in Spain were generated from responses to the 2006 National Health Survey5 and the 2009 European Health Survey.6 Three studies have analyzed mortality attributable to SHS in Spain. The first 2, published in 20027 and 2011,8 estimated that approximately 1000 never smokers aged ≥35 years died of ischemic heart disease or lung cancer each year. The third study, based on data for 2020, estimated that SHS caused 747 deaths a year, but in this case, the population included smokers, former smokers, and never smokers.9

Spain has 17 autonomous communities that operate with varying degrees of self-governance. More accurate estimates of mortality attributable to SHS in each of these regions would help identify priority areas for intervention. To our knowledge, no studies to date have calculated deaths attributable to SHS exposure in the different regions of Spain. The aim of this study was to estimate SHS-attributable mortality in both men and women aged ≥35 years in each of the country's autonomous communities for the period 2016 to 2021.

Mortality attributable to SHS was estimated using a prevalence-based method and population attributable fractions (PAFs), with adjustments for age group (35-54, 55-64, 65-74, and ≥75 years), sex, place of residence (autonomous community), and cause of death (ischemic heart disease or lung cancer). This study followed the STREAMS-p (Strengthen the design and REporting of Attributable Mortality Studies using a Prevalence-based method) guidelines.10

PAFs were calculated as:

where pSHS0 and pSHS1 are the respective prevalences of nonexposure and exposure to SHS, and RRSHS1 is the relative risk of death due to ischemic heart disease or lung cancer among exposed vs unexposed individuals.

For each group (defined by sex, age group, place of residence, and cause of death), mortality attributable to SHS was estimated by multiplying the observed mortality by the PAF: AM=OM × PAF.

Mortality attributable to SHS was also analyzed separately for never smokers, former smokers, and smokers. The corresponding equations are shown below:

Never smokers:

Former smokers:

Smokers:

where AM refers to attributable mortality and pnf,pexf and pf denote the respective prevalences of never smokers, former smokers, and smokers and RRexf and RRf the risk of death due to ischemic heart disease or lung cancer among smokers and former smokers vs never smokers.

We estimated crude and age-standardized attributable mortality rates. Age-standardized rates were calculated using the direct method with adjustment for sex and place of residence and application of the European standard population proposed by Eurostat's Task Force based on projections for the period 2011 to 2030.11

Attributable mortality rates with their 95%CIs were calculated using a naïve Bootstrap method and Efron's percentile method.

Between 2016 and 2021, exposure to SHS in Spain caused 4970 (95%CI, 4787-5387) deaths in adults aged ≥ 35 years. This corresponds to 1.6% of all observed deaths due to ischemic heart disease and lung cancer in that period. SHS was responsible for 3423 (95%CI, 3241-3742) deaths in men and 1547 (95%CI, 1434-1760) in women. In men, 2094 (95%CI, 1971-2302) deaths were due to ischemic heart disease (table 1), with 549 (95%CI, 502-618) occurring in never smokers, and 1400 (95%CI, 1293-1554) in former smokers. The respective figures for female never and former smokers were 927 (95%CI, 820-1073) and 210 (95%CI, 199-248) (table 2).

The burden of mortality due to ischemic heart disease or lung cancer attributed to SHS exposure varied geographically, with the lowest rates observed in the Principality of Asturias (0.7% for men and 0.6% for women) and the highest in Andalusia (3.6% for men and 2.6% for women). Ischemic heart disease was the main cause of death in men and women, regardless of place of residence. Crude mortality rates in men ranged from 46.6 deaths per 100 000 inhabitants in Andalusia to 10.7 deaths per 100 000 inhabitants in the Community of Madrid (table 1). In women, the rate ranged from 17.0 deaths per 100 000 inhabitants in Andalusia to 5.5 deaths per 100 000 inhabitants in Catalonia. The highest rates, in all regions, were observed in never smokers, particularly in the subgroup of women (table 2).

Age-standardized mortality rates for men ranged from 56.1 deaths per 100 000 inhabitants in Andalusia to 12.8 deaths per 100 000 inhabitants in the Community of Madrid. For women, they ranged from 18.2 deaths per 100 000 inhabitants in Andalusia to 5.4 deaths per 100 000 inhabitants in the Basque Country (figure 1 and table 1). Both crude and age-standardized rates differed between the two 3-year periods analyzed (table 5 of the supplementary data).

Figure 1.

Central illustration. Age-standardized mortality rates for lung cancer and ischemic heart disease attributable to exposure to secondhand smoke by sex and autonomous community.

(0.22MB).

Sensitivity analysis

Application of the RRs used in other studies analyzing mortality attributable to SHS in Spain yielded a total of 6134 (95%CI, 5923-6624) deaths; this is 23.4% higher than the figure obtained in our analysis.

When COPD and cerebrovascular disease were included in the analysis, the number of deaths attributable to SHS exposure between 2016 and 2021 increased to 9579 (95%CI, 9290-10 240). The greatest increase was observed for Andalusia, with age-standardized mortality rates increasing from 56.1 to 112.5 deaths per 100 000 inhabitants for men and from 18.2 to 38.0 deaths per 100 000 inhabitants for women (figure 2).

Figure 2.

Mortality rates for lung cancer, ischemic heart disease, chronic obstructive pulmonary disease (COPD), and cerebrovascular disease attributable to exposure to secondhand smoke. The rates are standardized by age and shown by autonomous community and sex.

(0.26MB).

Between 2016 and 2021, exposure to SHS in Spain caused approximately 5000 deaths due to ischemic heart disease or lung cancer and more than 9000 deaths due to ischemic heart disease, lung cancer, COPD, or cerebrovascular disease. The burden of mortality attributable to SHS exposure varied regionally, with the highest rates observed in Andalusia.

Although mortality attributable to SHS has been previously studied in Spain, ours is the first study to estimate rates for each of the country's 17 autonomous communities. Our findings confirm the existence of regional variations, although consideration must be paid to differences in the number of inhabitants and age structure. Andalusia and Catalonia, for example, with 18% and 16% of the country's total population respectively, have the largest populations. SHS-attributable ischemic heart disease and lung cancer deaths, however, were much higher in Andalusia than in Catalonia (33% vs 9%). Regional variations in age structure should also be taken into account, as they, rather than differences in SHS exposure, might explain differences in mortality rates. Spain has an overall aging index of 129.1, but regional indices range from 90.3 in the Region of Murcia to 231.1 in the Principality of Asturias. Andalusia and Catalonia have some of the youngest populations, with indices of 107.8 and 120.1, respctively.22 Nonetheless, when mortality rates were standardized for age, Andalusia still had the highest number of SHS-attributable deaths in both men and women. The age-standardized mortality rate in Andalusia, at 56.1 deaths per 100 000 inhabitants, was 4-times that of the region with the lowest rate, the Community of Madrid, with 12.8 deaths per 100 000 inhabitants. These variations can largely be explained by regional differences in exposure to SHS. In Andalusia, for example, more than 25% of men and women aged <65 years reported being exposed to SHS. The corresponding rate in the Community of Madrid was around 10%. These differences are likely reflections of differences in lifestyle habits and compliance with smoking laws. Andalusia, for instance, has one of the highest smoking rates and number of bars per 100 000 inhabitants in Spain, while Madrid has one of the lowest smoking rates and the fewest bars per 100 000 inhabitants.23,24 We were unable to compare regional levels of compliance with smoking laws, as the surveys used did not specify site of exposure.

Supporting previous findings for Spain, mortality attributable to SHS was higher in men than women. This finding was expected as men also had higher SHS exposure and observed mortality rates. Observed mortality rates were higher in men for all the conditions analyzed except cerebrovascular disease, and the difference was particularly evident in the case of lung cancer and COPD.17 Smoking is the main risk factor for both these diseases, and it should be noted that women started smoking significantly later than men in Spain.23

Another notable finding is the significant impact of SHS exposure on death due to ischemic heart disease and lung cancer in women who had never smoked: 60% of all SHS-attributable deaths in women occurred in never smokers. The corresponding rate for men was just 16%. The disparate results are probably linked to differences in smoking habits, as in Spain, more women than men have never smoked.23

It is not easy to compare our results with findings from other countries. In the past 5 years, just 3 studies that are strictly comparable in terms of causes of death and age have been conducted in other countries (China,25 the United States,26 and Morocco27). The Moroccan study reported an SHS-attributable mortality rate of 0.63% for individuals aged ≥35 years. This is higher than the rate of 0.19% detected for Spain in 2021 (data not shown), possibly because of differences in SHS exposure rates, lifestyle habits, and time periods analyzed (the Moroccan study analyzed data for 2012).

SHS exposure causes approximately 2 avoidable deaths every day in Spain. Our findings, however, show that the effects vary regionally and indicate that both the central government and regional authorities should legislate accordingly to reduce exposure, particularly in areas with high rates of SHS-attributable mortality. Successful initiatives implemented in other places, such as California, should be analyzed. Banning smoking in public areas34 such as beaches and state parks,35 cars when children are present,36 and multidwelling units37 would strengthen protection against involuntary exposure to tobacco smoke and heighten awareness among the general public of the importance of reducing exposure to SHS, which is a significant risk factor for numerous life-threatening diseases, in particular ischemic heart disease.

This study was funded by the Instituto de Salud Carlos III (ISCIII) via project PI22/00727 and cofunded by the European Union. It also received funding through a SEPAR 2023 grant (Project 1426).

This study was conducted in accordance with the principles for international medical research outlined in the Declaration of Helsinki. Ethical approval and informed consent were not necessary, as the study did not involve the participation of humans (or identifiable human material or data) or interventions involving humans. Sex and gender issues were taken into consideration in the study design and analysis.

No artificial intelligence tools were used in the preparation of this manuscript.

J. Rey-Brandariz: conception, data acquisition, interpretation of results, editing and critical review of manuscript; C. Guerra-Tort: data analysis, interpretation of results, editing and critical review of manuscript; D.C. López-Medina and G. García: data acquisition, interpretation of results, and critical review of manuscript; A. Teijeiro, R. Casal-Fernández, and C. Candal-Pedreira: interpretation of results and critical review of manuscript; L. Varela-Lema and A. Ruano-Ravina: conception, interpretation of results, and critical review of manuscript; M. Pérez-Ríos: conceptualization, acquisition of funding, drafting and editing of manuscript. All authors have read and approved the final version of the manuscript.

The authors declare that they have no conflicts of interest.