How Air Pollution Affects Breast Cancer

Introduction

Breast cancer is the leading cause of cancer-related deaths in Taiwan, and its prevalence is continuing to grow (Basic Information About Breast Cancer, 2021, Taiwan Ministry of Health and Welfare, 2016). Often in Taiwan, people are unaware of the onset of breast cancer since it appears suddenly and without a distinct cause, especially in women aged 40 to 74 (Klarenbach et al., 2018). One urban risk factor has come under specific scrutiny by Taiwanese scientists--air pollution (Hung et al., 2012). Coal-burning power plants and traffic exhaust emit a tremendous amount of pollutants such as polycyclic aromatic hydrocarbons (PAHs), and particulate matter 2.5 (PM2.5), which scientists suspect is the primary inducer of cancers (Hsu et al., 2019, Li et al., 2021, A Study on the Correlation between Breast Cancer and Air Pollution, 2017, Hung et al., 2012, Su et al., 2019). The following literature review aims to determine if air pollution is an environmental risk factor for breast cancer in women aged 40 above in Taiwan by examining the sources of air pollution, comparing research findings, and examining energy source shifts throughout the country. Currently, there are conflicting views on whether air pollution is a risk factor for breast cancer in Taiwan. However, clarification through this review will help drive scientists towards consensus and prompt action.

Hypothesis

The Hypothesis of author of the research paper hypothesize that several environmental factor such as PM 2.5 would have a significant impact on Breath Cancer, however the cause of the breath cancer should not only be considered on environmental factor, several inherited factor may also contribute to breath cancer, such as individual gene, or mutation in the allele may also be the cause of the Breath cancer. However PM2.5 would increase the chance of getting diagnosed with breath cancer.

Background Information

The incidence rate of breast cancer has been increasing globally (Li et al., 2021). Moreover, cancer is a leading cause of death worldwide, ranking second in 2019 (GBD Compare, 2019). According to the World Health Organization (WHO), a total of 2.3 million breast cancer cases occurred in 2020 (Cancer, 2021). In Taiwan, the incidence of breast cancer has nearly doubled in recent decades (Chang et al., 2012), while the mortality rate has increased from 54.79% to 149.78% from 1971-2010 (Ho et al., 2014). Despite this notable change in incidence and mortality, the known risk factors for breast cancer have fundamentally stayed the same and are related to women's reproductive systems and genetics (Hung et al., 2012). In addition, only one-third of new breast cancer cases are attributable to known risk factors, and many of the causes remain unexplained (Hung et al., 2012). This has led many experts to hypothesize that environmental exposures such as air pollution are contributing to the risk of breast cancer (Hung et al., 2012).

Ambient outdoor air pollution has been associated as a cause of many health problems such as cancer (Li et al., 2021). Air pollution is a complex mixture of chemicals and intoxicating particles harmful to the human body through inhalation or contact. Currently, in Taiwan, the two main sources of air pollution include coal-burning power plants and car exhaust (Li et al., 2021, A Study on the Correlation between Breast Cancer and Air Pollution, 2017). Air pollution from motor vehicle exhaust has been one of the most studied environmental factors. Exhaust from traffic is a complex mixture of many chemical compounds, including benzene, polycyclic aromatic hydrocarbons (PAH), and particulate matter 2.5 (PM 2.5). PM 2.5, in particular, contains more carcinogenicity and molecular weight compared to other pollutants. For example, exposure to PM2.5 can lead to inflammatory injury, immune response stress, oxidative DNA damage, DNA methylation, and insufficient DNA repair (Gichner, 1991, Gichenr, 1990). These genotoxic effects may increase cancer risk. Studies have accentuated PM 2.5 as the primary pollutant that can cause breast cancer in Taiwan (Hung et al., 2012).

Conflicting views still occur as other scientists find it hard to determine the biological connection between breast cancer and air pollution. Pollutants are inhaled and deposited in the lower respiratory tract; thus, they have no direct carcinogenicity to breast cancer. It is difficult to define a significant exposure measure of air pollution and breast cancer when the biological mechanisms of air pollution on breast cancer are largely ambiguous.

Methods

PubMed, Google Scholar, and NCBI were searched to identify the sources using the following search terms: "cancer," "air pollution," "Taiwan," and variation of "breast cancer," including "breast neoplasm," "ductal carcinoma," and "lobular carcinoma." The search queries used were: "Taiwan air pollution data," "Taiwan breast cancer incidence," and "(female) OR (middle-age) OR (breast cancer OR breast neoplasm) AND ("Air pollution") AND ("Taiwan")." Sources were included based on the following criteria: the test subjects were over 40 years of age, the articles were published after 2000, the results included quantitative results of the incidence of breast cancer in Taiwan after 2000 and air pollution data from specific atmosphere monitoring stations throughout Taiwan, and participants were traced annually to determine the ratio of the correlation between breast cancer and air pollution. Sources were excluded based on the following criteria: the environmental causes were not about air pollution, secondary sources, and papers published before 2000.

Independent Variable, Dependent Variable, and Constant

The independent variable of this study is the amount of PM 2.5 in the atmosphere which is the atmosphere in Taiwan, the higher PM2.5 level city which includes Kaohsiung City, Yunlin County, Chiayi County, and Pingtung County, and other lower PM 2.5 level city in Taiwan(Li et al., 2021, Su et al., 2019), The independent Variable of this study focused on how much people who is diagnosed with breath cancer, and the constant of this study is the age group of people and the gender of the people who is observed in the experiment, which is postmenopausal women over the age of 40.

Results

This literature review focuses on five studies. Four studies are retrospective chart reviews, and one study is a prospective interventional study. Three studies focus on the correlation between breast cancer and air pollution in Taiwan, including data on air pollution concentration and incidence of breast cancer provided by the Taiwan Cancer Registry database (Hsu et al., 2019, Li et al., 2021, A Study on the Correlation between Breast Cancer and Air Pollution, 2017). One study focuses mainly on the geographical variation in Taiwan and where in Taiwan has the highest incidence of breast cancer because of air pollution (Su et al., 2019). The fifth study focuses particularly on PM2.5 as the main pollutant with the most likely cause of breast cancer in Taiwan (Hung et al., 2012). All studies focused on postmenopausal women over the age of 40, conducted research in Taiwan, and used air pollution monitor stations in Taiwan for data collection.

PM 2.5 as a source of air pollution

Throughout the studies, PM2.5 concentration has the highest association with the incidence of breast cancer. In Taiwan, the primary sources of PM2.5 include industrial activities, such as coal-burning power plants and petrochemicals, fugitive dust from roads and construction works, and traffic-related exhaust emissions (Li et al., 2021). In order to understand the composition of air pollution, studies used the Taiwan Emission Data System (TEDS) implemented by the Taiwanese government in 1992. According to the studies, the recent report of TEDS shows that the primary sources of PM2.5 were fugitive dust from roads and construction works, traffic-related exhaust emissions, and industrial emissions, which accounted for 41%, 23%, and 22%, respectively, of the total emissions (Li et al., 2021). Major cities in Southern Taiwan, including Kaohsiung City, Yunlin County, Chiayi County, and Pingtung County, were attributable to the highest levels of PM2.5 emissions (Li et al., 2021, Su et al., 2019). The findings of TEDS have added to the growing evidence of the negative consequence of PM2.5 on health.

According to a cohort study in the United States exploring the association of PM2.5 with mortality from 29 cancer sites, bladder and kidney cancers had significantly positive associations (Turner, 2018). Another cohort study found that exposure to PM2.5 may increase liver cancer incidence in Europeans (although not statistically significant). Furthermore, studies in Taiwan reveal that exposure to high PM2.5 levels was significantly associated with increased lung, liver, breast, oral, and ovarian cancer (Pedersen et al., 2016). The studies also further examined the associations of PM2.5 with cancer types in developing, general, and aged towns and found significant positive correlations with oral, colorectal, liver, and skin cancers (Hung et al., 2012).

Aside from PM2.5, the study also indicated significantly positive correlations between other air pollutants (PM10, SO2, NOx, and O3) and age-adjusted cancer incidence rates (Hung et al., 2012). According to the report of TEDS, 93% and 89% of the total emissions for PM10 and NOx, respectively, were attributable to fugitive dust from roads and construction works, traffic-related exhaust emissions, and industrial emissions (Li et al., 2021). Studies have also suggested that air pollution due to pollutants other than PM2.5 is associated with an increased risk of lung, breast, prostate, and ovarian cancers (Hsu et al., 2019).

PM2.5 is considered the most relevant measure for biological effects of air pollution because these fine particles are respired into the lower respiratory tract. The findings should not be interpreted as meaning that PM2.5 is a causal factor; it is more likely a marker of the complex mixture derived from motor vehicle exhaust, coal-burning power plants, and many other factors. Overall the studies provide substantial evidence that PM 2.5 can most likely cause breast cancer.

Conflicting Findings

The studies have inconsistent results as some believe there is a correlation between air pollution and breast cancer in Taiwan (Li et al., 2021, Su et al., 2019, A Study on the Correlation between Breast Cancer and Air Pollution, 2017). In contrast, some authors say otherwise (Hung et al., 2012). Even though the studies included in this review all use data from the Taiwan Cancer Registry database, the difference in collection and extraction of the data caused variations in the studies’ findings. According to the prospective chart, the number of breast cancer incidences recorded is sectioned into four quartiles spanning from 2000 to 2018 (Li et al., 2021, Su et al., 2019). The adjusted hazard ratios of breast cancer after exposure to the Quartile 2, Quartile 3, and Quartile 4 levels were compared with those after exposure to the Quartile 1 level. With an interquartile range increase in each pollutant. Compared to the lowest quartile of each air pollutant, subjects exposed to the highest quartile (Quartile 4) had a significantly higher risk of breast cancer. For example, subjects exposed to carbon monoxide (CO) have an increase in breast cancer incidence, starting from 189 subjects diagnosed with breast cancer and increased to 389 subjects (Li et al., 2021, Su et al., 2019). As a result, studies conducted during the first and second quartile present different results to the correlation of breast cancer and air pollution than studies conducted in the third and fourth quartiles.

Several factors may contribute to the contradiction in research findings, including the differences in research data sources and the time the studies are conducted. The traditional disease studies mostly adopted regional data or performed research and exploration on certain diseases regarding regional data. Although they may help identify the leading cause of cancer, they did not employ an interactive approach with sound visual effects, which made the data easier to interpret. Researchers failed to give the health hazard extent caused by external environment factors in counties and cities, and they merely compared the regional changes at different time intervals. Because of this, Taiwan Cancer Registration Information was used to link with the air quality information and the density information of cars and motorcycles by newer studies which presented the results in a visualization manner (Su et al., 2019, A Study on the Correlation between Breast Cancer and Air Pollution, 2017). Besides performing statistical analysis from traditional disease studies to determine if vehicle exhaustion was related to the prevalence of breast cancer, visualization was also used to explore if there was other hidden critical information to learn about the impact of air pollution on breast cancer. Hence, more recent studies are more likely to find a higher affiliation between breast cancer and air pollution.

Another factor is that studies before 2010 did not focus on PM 2.5 as a pollutant that causes breast cancer (Li et al., 2021). An extensive air pollution monitoring network in Taiwan is managed by the Taiwan Environmental Protection Administration, which routinely monitors five"criteria" pollutants (PM10, O3, CO, SO2, NO2). The monitoring stations were fully automated and provided daily readings of pollutant levels. However, studies conducted before 2010 did not consider PM 2.5 as the primary source of air pollution in Taiwan (Hung et al., 2012). It was not until 2011 that PM2.5 concentrations in Taiwan were measured continuously. The monitoring stations provided data for PM2.5 in mean daily mass concentration (μg/m3 ). The monitoring network availability for PM2.5 offers the opportunity to investigate the impact of fine particle air pollution on female breast cancer incidence, revealing more detrimental health risks of PM 2.5 and a higher correlation between breast cancer and air pollution (Li et al., 2021).

Shifts in energy sources

Studies have shown an increase in the amount of PM 2.5 and other pollutants (PM10, O3, CO, SO2, NO2) in Taiwan. More recent studies have discovered an increased risk of breast cancer because the air quality in Taiwan is gradually deteriorating every year. Aside from the heavy traffic congestion in Taiwan, coal-burning power plants pose the greatest air pollution emission in Taiwan. The transition from nuclear power to coal-burning is the main reason why there is a pattern of increase in air pollution in the studies. Natural gas and coal burning have gone from generating 135,657kw in the early 2000s to 229,815kw in 2018, while nuclear power declines from 42,117kw to 27,682kw (台灣能源, 2021). The shift in energy sources alters the results of the studies over time and where the data of most studies are extracted from.

Most air pollution data are derived from monitoring stations in the middle and southern parts of Taiwan because the major coal-burning power plants are located there. According to the studies, the two clusters of PM 2.5 93%, 90%, 86%, and 81% of the total emissions are Kaohsiung City, Yunlin County, Chiayi County, and Pingtung County (Su et al., 2019). Another study shows prospective charts that indicate the air pollutants and breast cancer trends in Taiwan's cities, which also shows similar results as the top three cities that exposed to an average higher concentration of air pollutants PM2.5 are Kaohsiung City, Chiayi City, and Tainan City (A Study on the Correlation between Breast Cancer and Air Pollution, 2017). The top three cities exposed to a standard higher concentration of NO2 are Taipei City, New Taipei City, and Kaohsiung City (A Study on the Correlation between Breast Cancer and Air Pollution, 2017). This is because of the heavy traffic congestion in Taipei, Taiwan's capital, and Kaohsiung city, the second-largest city in Taiwan.

A higher concentration of PM 2.5 is likely to occur in the middle and southern part of Taiwan because of the shift in energy sources from nuclear power to coal-burning power. With the help of visual observation from the studies, cities exposed to an average higher concentration of pollutants PM2.5 and NO2 have a relatively higher incidence of breast cancer (Li et al., 2021, Su et al., 2019, A Study on the Correlation between Breast Cancer and Air Pollution, 2017, Hsu et al., 2019). Therefore, the transition in energy sources has a significant impact on the studies results of the topic.

Conclusion

With the progress of urbanization in Taiwan, the effect of environmental air pollution becomes more and more serious, and the incidence rate of cancer in Taiwan is also rising. Ambient air pollution may be an essential factor in the increase of breast cancer incidence. Research into environmental chemical exposure helps promote public health benefits. Nevertheless, there are limitations to the studies. As studies conducted on PM 2.5 look at more than just breast cancer, the results that are not focused on breast cancer can affect the research process. Also, older studies lack advanced scientific technology and government data transparency, so the results may deviate from studies conducted more recently. Although there are limitations, and the risk factors that increase cancer are numerous and complex, the studies provide several potential answers, including the emission of pollutants from traffic and industrial exhaustion. According to research statistics, some air pollutants are closely related to the risk of breast cancer, such as PM 2.5. This should be the heart of the issue and be tackled by scientists to fix air pollution problems and cease the increased cancer rate in Taiwan.