To our knowledge, this is the first study that estimated the global cancer burden attributable to secondhand smoke. Our study found that age-standardized rates of cancer deaths, DALYs, and YLLs decreased significantly over the past three decades, but the absolute numbers are consecutively increasing, and the cancer burden due to secondhand smoke share in the total cancer burden has tended to increase in recent years. In terms of the distribution of cancer burden by demographics, older individuals and women incur a greater cancer burden attributable to secondhand smoke. From the regional level, it is revealed that there is a significant difference in the status quo and trend of attributable cancer burden across regions. In 2019, the concentration of cancer burden was seen in European countries, followed by North America and Oceania. Nevertheless, the latter two regions experienced decreases in cancer burden from 1990 to 2019. In contrast, Africa and Asia, which had lower cancer burdens in 2019, experienced increases in the annual rate changes in deaths, DALYs, YLDs and YLLs. For cancer burden by SDI, high-middle SDI countries and middle SDI countries had higher age-standardized rates of deaths, DALYs, YLDs and YLLs than the global level in 2019. Reduction in cancer burden caused by secondhand smoke is more encouraging in higher SDI countries. These findings pose implications to improve population health and reduce avoidable health loss.
Cancer burden attributable to secondhand smoke at the global level
According to the results, secondhand smoke climbed up to the tenth leading risk of cancer DALYs, resulting in increases in the absolute numbers of cancer deaths, DALYs, YLDs and YLLs, particularly in YLDs number, which doubled by 2019, and the rising annual proportion of cancer burden due to secondhand smoke in recent years. Additionally, there were reductions of 13%, 20% and 21% in the rates of deaths, DALYs and YLLs, respectively. These results are in consistence with the findings of a previous study that investigated cancer burden for all risk factors combined over thirty years.18 This seeming paradox could, to some extent, be explained by the growth of the population and aging. A world population project led by the UN showed that the global population has been growing by over 1.0% per year, and life expectancy has been increasing from 64.5 in 1990 to 72.4 in 2019 globally.20 As a result, there is a change in age structure that gives rise to a larger size and proportion of the older all over the world.21 Cancer prevalence is much higher in older people than the young; for example, approximately half of the total cancer cases are aged 70 years or older in America.22 Therefore, there is an increase in the number of cancer deaths but a decline in cancer mortality.
Furthermore, improvements in access to medical services might lead to an increase in prevalence and disability in cancer survivors who were exposed to secondhand smoke. Thanks to advances in cancer detection and treatments, many patients can be diagnosed at an early stage and receive life-sustaining therapy.23 This is also the reason why there was a dramatic increase in cancer-related YLDs number. A previous study documented that the 5-year survival rate for patients with cancer improved for different types of cancer from 1990 to 2009,22 implying that life years of the cancer patients lives with disability increased as well. Given the increases in absolute numbers and annual contribution of cancer burden attributable to secondhand smoke, there are challenges for health care systems that have not been well prepared for population growth and aging.24 It is highly recommended to optimize medical services, such as promoting cancer screening for those at high risk and improving follow-up care for older individuals. In addition, infrastructure should be developed that can meet the needs of growing cancer survivors.
Cancer burden attributable to secondhand smoke by age and sex
In terms of the distribution of attributable cancer burden by demographic characteristics, older individuals incurred a greater burden in 2019. More than half of the cancer burden was concentrated in adults aged 55–75 for men and in adults aged 50–70 for women. The age-standardized rates of deaths, DALYs, YLDs and YLLs rose with age for both sexes. As mentioned above, a greater cancer burden in older individuals could be traced to population growth and aging. Additionally, older people are vulnerable to secondhand smoke exposure and have a cumulative risk of exposure to secondhand smoke, so they are more affected by tobacco once they have cancer. In addition, older people commonly have other morbidities that exacerbate cancer prognosis.25 According to a report by the United States Centers for Disease Control and Prevention (CDC), approximately 80% of adults aged 65 years and older have at least one chronic condition, and 50% have at least two chronic conditions.26 Chronic conditions could deteriorate the health loss caused by cancer.
Although there were declines in cancer mortality, DALYs rate, YLDs rate, and YLLs rate among other age groups in 2019 compared to those in 1990, older individuals experienced increases in cancer burden. Similarly, a study that investigated disease burden for all diseases combined revealed that age-standardized all-cause YLD rates are higher in older ages.9 As discussed, the proportion of older individuals was higher in 2019 than in 1990 because of the growth of aging.21 The advances in cancer detection and treatments and improvements in access to medical services might have led to the increase in prevalence and disability in cancer survivors who were exposed to secondhand smoke. In addition, it is more likely for them to develop other NCDs disease due to changes in lifestyle behaviors, such as poor physical activity, which also increase cancer risk and trigger more disease burden.23 Once cancer occurs in the older, they may be more fragile, and their health might deteriorate rapidly.25 Increasing burden in the older has important implications for health policy, including protecting the older from carcinogenic risks, ensuring earlier cancer screening, and providing long-term supportive care for older patients with cancer.25
In terms of sex differences, the gap in cancer burden attributable to secondhand smoke consecutively narrowed, resulting from a substantial decline among men and a slight increase among women. Notably, the age-standardized YLDs rate continued to rise among women, which was contrary to men. This pattern reflects that the efforts on smoking restrictions and cancer control do not equally bring the same benefits to men and women, although women are more likely to secondhand smoke and have an approximately 30% higher risk of cancer owing to secondhand smoke.5 To some extent, this implies that the effect of health policies on women is not as favorable as on men, potentially reflecting inequalities in cancer prevention, intervention and treatment across genders over the past thirty years.9 The underlying reasons for this inequality might be complex. Further studies are needed to explore the sex-specific distribution of attributable cancer burden and the disparity of cancer-related health policies in men and women.
Notably, women under 75 years had a significantly higher YLDs rate than men of the same age group in 2019, suggesting that health loss due to cancer is more common in women. One explanation for this is that women might be more attentive to disorders so that they actively seek treatment. In this sense, life time with cancer disability might be longer for women. Gender differences in cancer-related health loss should be further examined in future studies.
Cancer burden attributable to secondhand smoke by geographic region
In regard to the distribution of cancer burden across different regions, the concentration of cancer burden attributable to secondhand smoke was in Europe in 2019, particularly in countries from the Balkan Peninsula. Furthermore, the Balkan Peninsula also had a greater annual rate change in terms of cancer deaths, DALYs, YLDs and YLLs. This finding is consistent with previous studies.18,23 It is probably driven by high tobacco prevalence, high cancer incidence and rapid aging. According to a WHO report, the prevalence rate of current tobacco use in Europe is nearly as high as the global level and just second to the South-East Asian in 2019; a relatively slower prevalence rate of decline was seen in the European region from 2000 to 2019.27 More than 20% all-cause DALYs were attributable to tobacco use in the countries of Balkan Peninsula in 2019; more than 10% DALYs were attributable to tobacco use in the majority of other European countries.18 On the other hand, Europe has higher cancer incidence for both sexes combined, and cancers that are related to much greater disease burden are more common in Europe.23 For example, the incidence rate and mortality of breast cancer in Europe are relatively higher than those in other regions, while women are the main vulnerable population to breast cancer as well as secondhand smoke. Additionally, the UN project revealed that the European population is growing at a slow rate of 0.3%, but aging is rather rapid in most European countries,20 in which the proportion of adults aged over 65 years will reach 24.3% by 2030.26 This might also be a reason for the increase in the annual rate change of the attributable cancer burden.
Different from Europe, some countries in North America and Oceania had higher rates of cancer deaths, DALYs, YLDs and YLLs in 2019, yet the annual rate change of cancer burden decreased mainly in these two regions. The greater cancer burden due to secondhand smoke in 2019 might be explained by relatively high tobacco use and cancer incidence. The tobacco prevalence in North America and Oceania was lower than that in Europe in 2019, resulting in only 6%-8% of all-cause DALYs being attributable to tobacco use in both regions, but it is still higher than that in other regions.18,27 Cancer incidence was also high in North America and Oceania;23 for example, the breast cancer incidence rate is 91.6 per 100,000 in North America and 85.8 per 100,000 in Oceania. On the other hand, consistent with the global trends in population growth, the aging population is increasing in these regions as well.20 As a result, the attributable cancer burden in North America and Oceania is not as high as that in Europe but is still higher than that in the other regions. The reduction in the annual change in cancer burden in North America and Oceania may be associated with a decline in smoking prevalence and optimization of cancer care. In Western countries, such as in the United States, cancer mortality is decreasing, particularly for cancer highly associated with tobacco use, since the tobacco epidemic started the earliest and peaked around the middle of the last century.28–30 In addition, there have been considerable achievements in cancer treatment since the last century, especially in developed countries in the West, which have improved the clinical outcomes of cancer patients and prolonged their lifetimes. For instance, in the US, the total 5-year survival rate of breast cancer increased by more than 15% from 1975 to 2009; the overall 5-year survival rate of prostate cancer increased by more than 30% from 1975 to 2009.22 Therefore, the secondhand smoke-related cancer burden tends to decrease annually.
Notably, Africa had lower rates of cancer deaths, DALYs, YLDs and YLLs than other regions in 2019, yet there was an increase in annual rate change in most African countries. An explanation for this pattern lies in the lower degree of cancer burden attributable to secondhand smoke at baseline in Africa. Thus, although the cancer burden is rising annually in Africa, it still had a lower cancer burden than other regions in 2019. Furthermore, tobacco prevalence is consecutively lower in Africa than in other regions over the past years.27 Nevertheless, due to the increasing growth in population, which doubled from 1990 to 2019 with an average annual rate of over 2.4%,19 African countries experienced an increase in the annual change of cancer due to secondhand smoke. Moreover, poor medical treatment and limited access to well-integrated survivorship care are also triggers of rising trends of cancer burden in Africa.31 To avoid continuing growth in cancer burden attributable to secondhand smoke, it calls for urgent action to improve health for he older and promote medical services for cancer patients.22,25
A rising annual rate change in cancer burden was also seen in many Asian countries, among which China stood out and had a relatively greater cancer burden attributable to secondhand smoke than other Asian countries in 2019. This finding might stem from several aspects. First, all DALYs caused by smoking in Asia are relatively high. According to GBD Study 2019, in many parts of Asia, tobacco exposure was attributed to 10–20% of DALYs for all causes combined, while in two provinces of China—Liaoning and Heilongjiang—attributable DALYs accounted for more than 20%, which was much higher than other locations.18 Second, during the past years, there was a large reduction in tobacco prevalence, which fell by more than 10%, yet the average prevalence (25%) was still much higher in the Asian region.30 In particular, the overall smoking rate among adults aged 15 years and older was 26.6% in China in 2018, 50.5% for men and 2.1% for women. More than 40% of the adults reported being exposed to secondhand smoke either at home or in other public places. 32 Third, Asia is the most populous region, accounting for a considerable percentage of the aging population worldwide.20 It was projected that adults aged over 65 years will be up to 12% in Asia by 2030. China, as the most populous country, is experiencing a decline in fertility and an increase in the average life span, resulting in an acceleration in the change in age structure.21 Last, limited medical resources and less mature cancer treatment potentially lead to unsatisfactory clinical outcomes.33 Regardless of medical resource distribution or advanced therapy for cancer patients, there are differences among China and other countries. In sum, health loss due to secondhand smoke is increasing annually in Asia, posing remarkable challenges to individuals’ health and health systems. In this sense, it is recommended to reverse the trends through sustained efforts, such as high taxation, smoking advertisement bans, and improvement in cancer screening and treatment.9,18
Cancer burden attributable to secondhand smoke by SDI
It was found that there were wide disparities in cancer burden attributable to secondhand smoke among the five SDI groups. Although age-standardized rates of cancer deaths, DALYs, YLDs and YLLs were higher in high SDI and high-middle SDI countries in 1990, they decreased significantly in high SDI countries, while they decreased slightly in high-middle SDI countries, resulting in a higher cancer burden attributable to secondhand smoke in high-middle SDI than the global level. In contrast, the cancer burden attributable to secondhand smoke increased slightly in other SDI countries, especially in middle-SDI countries, which surpassed the global level in later years. That is, the cancer burden decreases more significantly as the SDI level increases, which has also been discovered by previous studies.9,18,23 The drivers behind this finding could be complicated, as SDI is a compositive index.
Smoking is not largely correlated with SDI,18 which indicates that the imbalance in cancer burden is primarily stemmed from socioeconomic development and cancer control across countries. Approximately 50% of cancer cases are seen in high SDI countries, but only 30% of cancer deaths, 25% of cancer DALYs, and 23% of cancer YLLs occur in high SDI countries.9 A higher SDI generally reflects desirable social and economic status, which enables cancer patients to receive advanced treatment and incur less health loss.18 However, the greatest increase (52%) in cancer incidence was seen in middle SDI countries from 2007 to 2017.9 The cancer burden will shift to less developed countries due to population growth and aging and the increasing prevalence of risk factors.34
These findings imply an insufficiency in cancer prevention and treatment across less developed regions, which demands interventions to address health inequality worldwide.22 To address this international variation, recognizing the interdependencies between socioeconomic status and health is the first step.9 Lower SDI countries should make more efforts to promote smoking cessation and cancer prevention and control, such as raising tobacco tax, banning smoking in public places, forbidding tobacco advertisement, providing early cancer screening for those at high risk, and improving treatment and follow-up care for cancer patients.23 In addition, economic support should be provided to the patients for whom cancer treatment expenditures, including costs for both short-term and long-term care, might be catastrophic.
Two limitations of this study should be noted. First, the global burden of cancer burden attributable to secondhand smoke might be underestimated because data used in this study were directly extracted from the GHDx database, and data for some locations, such as Western Sahara (Africa), French Guiana (South America) and Svalbard (Europe), are not available. Although this study analyzed the data related to cancer burden attributable to secondhand smoke in most countries or regions of the world, it is warranted to collect more comprehensive data and conduct further analysis on the cancer burden attributable to secondhand smoke. Second, the disease burden of different types of cancer attributable to secondhand smoke was not assessed in this study. Data on all cancers associated with secondhand smoke are available in the database, whereas the data are not classified according to cancer types. Given the close association between different types of cancer and secondhand smoke, future studies are needed to explore disease burden due to secondhand smoke by cancer type in future studies.