The International Agency for Research on Cancer (IARC) has classified outdoor air pollution as a Group 1 carcinogen. Increasing evidence has suggested that the association may not be limited to lung cancer, but also cancer of other sites.
Indoor emissions from the household combustion of coal are a Group 1 carcinogen. The household combustion of biomass fuels and emissions from high-temperature frying have been concluded to be possibly carcinogenic to humans by the IARC. It is estimated that 3.8 million premature deaths (8% from lung cancer) are attributable to household air pollution from cooking with kerosene or solid fuels each year.
The Global Burden of Disease Study also suggested that indoor air pollution is associated with the risk of esophageal squamous cell carcinoma. The forgotten 2.6 billion people who have less access to cleaner fuels, mostly in low- and middle-income countries (LMICs), Kenya included suffer an increasing cancer burden and are also exposed to a huge amount of air pollution. The vicious cycle of climate change, air pollution, and cancer will keep rolling and provide a devastating cancer burden without intervention.
Cancer has become the leading cause of premature death in many countries in recent decades. Previous studies showed plenty of evidence that control of modifiable risk factors would reduce the cancer burden. Since modifiable risk factors could be eliminated by changing the lifestyles of individuals, a greater uptake of modifiable risk factors is critical to reducing cancer burden and inequality in cancer survival.
However, climate change has widen cancer inequities through its complex connections with modifiable risk factors which include; abnormal temperature, UV, air pollution, natural disasters, food (diet), water, infections, and inefficient physical activities.
The associations between climate change and modifiable risk factors have no doubt expanded the inequities. People who face overlapping modifiable risk factors, but who are unable to change or adapt, are at the highest risk in the climate change–cancer linkage. Individual actions to avoid exposure to modifiable risk factors is highly recommended, limited benefits would be achieved unless the nations strive to ensure the basic needs of the people.
Cancer is the second cause of globally ranked premature death after cardiovascular disease and has become the top cause in many countries in recent decades. Apart from lifestyle factors, air pollution and other environmental risks are also the main factors associated with cancer. The inequality that exists in overall cancer survival between different countries is predominantly due to the disparities in health awareness; early diagnoses; screenings; and accessible, affordable, and high-quality healthcare for patients with cancer.
Data from the United States suggests that 42% of cancer incidence and 45% of cancer mortality were attributable to modifiable risk factors, a kind of behavior or exposure factors related to cancer risk that we could reduce or control, such as poor diet and physical inactivity.5 Findings from a population-based cohort study suggested that modifiable risk factors could explain 46% of inequities in cancer morbidity and mortality.6 Inequities exist in cancer burden because cancer survival and mortality burden are closely related to the treatment ability and care. The reduction of cancer burdens attributable to avoiding modifiable risk factors would provide an equitable strategy for cancer prevention and control. Therefore, it is critical to provide a supportive environment and educational programs on how to avoid modifiable risk factors is critical. However, climate change is challenging this approach and will widen cancer inequality.
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Potential Critical Connections of Climate Change and Cancer Risk
The potential linkage between climate change and factors that can modify cancer incidence and threaten cancer survival are shown in Figure
Figure1
1. The combustion of fossil fuel seems to be the start of a vicious cycle since it increases greenhouse gas emissions (CO2, CH4, etc.) that aggravate climate change and produce air pollution. According to previous studies, cancers potentially associated with modifiable risk factors are highlighted in Figure
Figure2
2.
Figure 1
Potentially critical connections between climate change and cancer risk. UV: ultraviolet radiation; the dashed line indicates that high temperatures could amplify, or enhance, the effects of ambient air pollution. Natural disasters include heat waves, cold spells, wildfires, droughts, floods, hurricanes, typhoons, etc.
Figure 2
Potential associations between climate-change-related risk factors and cancer.
The International Agency for Research on Cancer (IARC) has classified outdoor air pollution as a Group 1 carcinogen. Increasing evidence has suggested that the association may not be limited to lung cancer, but also cancer of other sites.7 Increased incidence and shorter survival of cancer affected by air pollution were observed in previous studies.7 In addition to the greenhouse gases emitted by the combustion of fossil fuels contributing to climate change, climate change may worsen air quality by changing the ventilation, dilution, precipitation and other removal processes, and the amplification of atmospheric chemistry.8 Accumulative evidence suggests that climate change is expected to increase particulate matter ≤ 2.5 μm (PM2.5)-related mortalities even under the scenario of a moderate projection of greenhouse gas emissions.9 Though the interplay of climate change and air pollution on health has been well documented, the evidence for cancer warrants investigation.10
Besides outdoor air pollution, household air pollution is also a part of the linkage.11 There are still millions of people who are not able to cook with clean energy, which results in severe indoor air pollution as well as greenhouse gases, that is, the major factor affecting climate change.12 Indoor emissions from the household combustion of coal are a Group 1 carcinogen. The household combustion of biomass fuels and emissions from high-temperature frying have been concluded to be possibly carcinogenic to humans by the IARC. It is estimated that 3.8 million premature deaths (8% from lung cancer) are attributable to household air pollution from cooking with kerosene or solid fuels each year. The Global Burden of Disease Study also suggested that indoor air pollution is associated with the risk of esophageal squamous cell carcinoma.13 “The forgotten 2.6 billion people” who have less access to cleaner fuels, mostly in low- and middle-income countries (LMICs), suffer an increasing cancer burden and are also exposed to a huge amount of air pollution. The vicious cycle of climate change, air pollution, and cancer will keep rolling and provide a devastating cancer burden without intervention.
With the accumulation of greenhouse gases, global warming, and changing meteorological factors, there is an increasing frequency of abnormally hot and cold temperatures, which are the direct expression of climate variability. Although it is not clear whether it is the increased temperature of warm months, carcinomatous meningitis was observed to have a higher incidence in these months.14 Evidence from more studies is warranted to confirm the causal association. The direct effects of extreme temperatures like heat waves may worsen the conditions of cancer patients because of their impaired thermoregulation and immune function.15 Occupational heat exposure, exacerbated by climate change,16 is also associated with several cancers, e.g., female breast cancer.17 The climate-change-induced ozone depletion led to increasing ultraviolet radiation (UV) exposure, which will lead to increased risks of melanoma and other skin cancers (e.g., squamous cell skin cancer).18 In consequence, melanoma survivors have an increased risk of developing second primary cancers, such as prostate cancer.19
The changing temperature and precipitation will also lead to changes in natural disasters and food production, which may increase the risk of cancer indirectly. Take wildfires as an example of a natural disaster that has shown an increasing trend in frequency and duration globally.20 Long-term exposure to wildfires has been suggested to increase the incidence of lung cancer and brain tumors.21 Extreme weather and natural disasters caused by climate change may reduce people’s physical activity by limiting active venues and suitable temperatures. Less physical activity could increase the risks of colon, female breast, and uterine cancers.5 Furthermore, inactivity-induced excess body weight also increases the risk of cancers of the gastrointestinal tract, genital organs, and female breast.
Less food availability and deteriorating food security induced by climate change have been observed. A modeling study revealed that by 2050, the global food availability would decrease by about 3.2%, and a 4.0% fruit and vegetable consumption reduction would be attributable to climate change.22 Parental malnutrition and exposure to air pollution exacerbated by climate change are significantly associated with serious adverse pregnancy outcomes,23,24 which are associated with higher breast cancer risk in early adult life.25 Lower consumption of fresh vegetables and fruit would also increase the risk of cancer from the respiratory and digestive tracts. Polycyclic aromatic hydrocarbons (PAHs) from high-temperature pyrogenic processes and vehicular emissions, which are related to the higher risk of breast cancer and colorectal cancer,27 were also found in commonly consumed leafy vegetables.26 The increase in the temperature led by climate change may increase appetite suppression and reduce conception rate of livestock because of heat stress, as well as reduce milk production.28 Lower dairy product consumption has been observed in line with increased breast cancer and colorectal cancer.27 Temperature and moisture significantly influenced aflatoxin production.29 Aflatoxin B1 production in maize will have consequences on aflatoxin M1 contamination in milk30 and might increase the liver cancer risk.31 Similarly, the diversity, distribution, and biology of fish populations are also affected by climate change. It is estimated that for every 1 °C of global warming, global fish catches are expected to decline by 3 million tonnes.32 Liver and colorectal cancer risks will be increased27 due to the lack and unaffordable fish consumption, especially in low-income countries.33 Climatic variability has always been the main factor responsible for fluctuations in coffee productivity worldwide.34 Coffee consumption is suggested to reduce the risk of oral and liver cancers.27 Apart from food, water quality is also affected. Climate change is likely to exacerbate the increasing bromide levels in public water systems, which would increase the excess lifetime bladder cancer risk35 and the cyanotoxins in drinking water that are associated with liver and colorectal cancer.36
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