Pollution And Cancer: Is There A Link?

Air pollution has been linked to lung cancer, with evidence suggesting that it is the second leading cause of the disease after smoking. Outdoor air pollution has been classified as a class I human carcinogen by the World Health Organization (WHO). It is largely a human-made problem, resulting from sources such as manufacturing, energy production, farming, flying, driving, shipping, and more. Studies have also found a link between air pollution and cancers of the upper digestive tract, including the head, neck, and nasopharyngeal cancers. Additionally, there is some evidence to suggest that air pollution may be a possible cause of bladder, breast, liver, kidney, colon, rectal, and pancreatic cancer. However, more research is needed to confirm these findings.

Air pollution and lung cancer

Air pollution has been linked to lung cancer, with research showing that particle pollution in the outdoor air we breathe can cause this disease. This type of pollution includes a mix of tiny solid and liquid particles in the air, such as acids, organic chemicals, metals, soil, and dust particles. These particles are emitted directly from sources like wood stoves, forest fires, vehicles, and power plants.

In 2013, the World Health Organization (WHO) International Agency for Research on Cancer reviewed the available science and concluded that particulate matter causes lung cancer. A separate study in Hong Kong and Birmingham, United Kingdom, found that for every 10 micrograms per cubic meter (µg/m3) of increased exposure to PM2.5, the risk of dying from lung cancer rose by 36%. This is especially true for people who live in areas with high levels of PM2.5 pollution.

While smoking is a much bigger risk factor for lung cancer, outdoor air pollution is still responsible for a significant number of cases. In the United Kingdom, for example, outdoor air pollution causes approximately 1 in 10 cases of lung cancer, and an estimated 6,000 people who have never smoked die from lung cancer each year due to air pollution exposure. Globally, around 300,000 lung cancer deaths in 2019 were attributed to PM2.5 exposure.

The mechanism by which air pollution causes lung cancer is still being investigated. One theory suggests that the tiny particles in air pollution may build up in the lungs and damage the DNA in cells, changing how they divide and leading to cancer. Another theory proposes that air pollution causes inflammation in the lungs, activating normally inactive cells carrying cancer-causing mutations and triggering uncontrolled cell growth and tumour formation.

To reduce the risk of lung cancer from air pollution, individuals can check the air quality index forecast and limit their activity during high-pollution periods. Additionally, making more journeys by walking, cycling, or wheeling can help lower emissions compared to driving.

Water pollution and bladder cancer

Water pollution is a potential risk factor for bladder cancer. A number of studies have found a positive correlation between high fluid intake and an increased risk of bladder cancer. However, the relationship between fluid intake and bladder cancer risk is still a subject of debate, with some studies finding no significant association.

One theory, the urogenous contact hypothesis, suggests that increased fluid intake leads to higher urine volume and urination frequency, thereby reducing the concentration of potential carcinogens in the bladder and the time they are in contact with the bladder epithelium. However, if the water source is contaminated with carcinogens such as chlorination byproducts and arsenic, high-level intake may elevate the exposure of the bladder epithelium to carcinogens and increase the risk of bladder cancer.

A meta-analysis of 26 case-control and 5 cohort studies found a significant positive association between high fluid intake and bladder cancer risk in European males and American residents, but a protective effect in Asian populations. The authors suggested that the association between total fluid intake and bladder cancer risk was largely influenced by differences in water quality, drinking habits, and gender, with males being more susceptible to these environmental and lifestyle risk factors.

Another meta-analysis of 687 articles published between 1972 and 2023 in the Scopus database found that the number of publications on drinking water pollution and bladder cancer has been increasing, indicating growing interest in this topic. The United States had the largest number of published articles, institutions, and funding sponsors. Arsenic was the most frequent contaminant mentioned in the articles, followed by chlorination, trihalomethane, and disease agents. The authors of this study also suggested that there is a regional imbalance in the available information on drinking water pollution and bladder cancer, with traditionally developed countries conducting most of the research.

Air pollution and breast cancer

Air pollution has been linked to an increased risk of developing lung cancer. However, research has also found a link between air pollution and breast cancer.

A study by the National Institutes of Health (NIH) found that living in an area with high levels of air pollution was associated with a higher incidence of breast cancer. The study, which was published in the Journal of the National Cancer Institute, was one of the largest of its kind, with over 500,000 participants from six US states and two metropolitan areas. The researchers found an 8% increase in breast cancer cases among women exposed to higher levels of fine particulate matter (PM2.5) near their homes. PM2.5 refers to particulate matter with a diameter of 2.5 microns or smaller, which can be inhaled deep into the lungs. These particles come from various sources, including vehicle exhaust, combustion processes, wood smoke, and industrial emissions.

Another study from Hong Kong and the UK, with 66,280 participants, found that long-term exposure to PM2.5 was associated with an 80% higher mortality risk from breast cancer. This study also showed that for every 10 micrograms per cubic meter (µg/m3) increase in PM2.5 exposure, the risk of dying from any type of cancer rose by 22%.

The exact mechanisms by which air pollution increases the risk of breast cancer are still being investigated. However, researchers have suggested that pollution may disrupt DNA repair functions, alter the body's immune response, or trigger inflammation that promotes tumour growth.

While air pollution is just one of several risk factors for breast cancer, these findings highlight the importance of reducing pollution levels to protect public health. Further research is needed to fully understand the relationship between air pollution and breast cancer risk, especially in different geographical areas.

Air pollution and pancreatic cancer

Air pollution has been linked to an increased risk of developing lung cancer. A recent study has also found a link between air pollution and pancreatic cancer.

A study of 66,280 residents of Hong Kong, all aged 65 or older, found that for every 10 microgram per cubic meter (µg/m3) of increased exposure to PM2.5, the risk of dying from any cancer rose by 22 percent. For cancers of the accessory digestive organs, which include the liver, bile ducts, gall bladder, and pancreas, the mortality risk was 35 percent higher.

Another study of 100,527 men and women from the Multiethnic Cohort Study, residing in Los Angeles County, found that PM2.5 (per 10 µg/m3) was associated with incident pancreatic cancer (hazard ratio [HR] = 1.61; 95% CI, 1.09, 2.37). This association was strongest among Latinos (HR = 3.59; 95% CI, 1.60, 8.06) and ever-smokers (HR = 1.76; 95% CI, 1.05, 2.94).

The findings suggest that air pollution, particularly fine particulate matter (PM2.5), may be a risk factor for pancreatic cancer, especially among certain racial and ethnic groups, as well as smokers. However, more research is needed to confirm these associations and to understand the underlying mechanisms.

Air pollution and DNA damage

Air pollution has been linked to DNA damage, which is a prerequisite for most mutations and cancers. Outdoor air pollution, in particular, has been found to cause DNA damage. This is supported by studies that show how outdoor air exposures increase the risk of a variety of diseases, such as asthma, heart disease, and lung cancer.

A growing body of research indicates that air pollution exposure modulates the epigenetic mark, DNA methylation (DNAm), and that these changes might influence inflammation, disease development, and exacerbation risk. Several traffic-related air pollution (TRAP) components, including particulate matter (PM), black carbon (BC), ozone (O3), nitrogen oxides (NOx), and polyaromatic hydrocarbons (PAHs), have been associated with changes in DNAm. Effects of air pollution on DNAm have been observed across the human lifespan, but it is not yet clear whether early life developmental sensitivity or the accumulation of exposures have the most significant effects on health.

A study in Fresno, California, found that children and teens exposed to high levels of traffic-related air pollution had evidence of a specific type of DNA damage called telomere shortening. The study also found that young people with asthma had evidence of telomere shortening and were exposed to higher levels of PAHs than those without asthma.

Another study examined the impact of air pollution on DNAm and found that black carbon exposure was associated with significantly higher DNAm in nasal brushings at cg23602092 in the promoter region of TET1. There was also a significant association between asthma status and DNAm at this site, with lower mean methylation in asthmatic children.

While the exact mechanisms through which air pollution modulates DNAm are not yet fully understood, oxidative stress induced by reactive oxygen species (ROS) is believed to play a central role in the downstream effects of inhaled air pollution. Oxidative species may also reduce the expression of methionine adenosyltransferase 1A (MAT1A) and the efficiency of the one-carbon metabolism pathway, leading to a scarcity of the methyl donor SAMe needed to establish and maintain DNAm.

In summary, air pollution has been linked to DNA damage, and this damage may occur through various mechanisms, including oxidative stress and the formation of reactive oxygen species. Further research is needed to fully understand the impact of air pollution on DNA damage and the development of effective interventions to reduce this risk.

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