In some regions, mouth cancer incidence has risen. A recent study uncovers a new risk factor that might help explain these increases.
In certain parts of the world, over the past couple of decades, mouth cancer rates have soared.
For instance, in the United Kingdom, rates of mouth cancer have increased by 68 percent. They rose from eight cases per 100,0000 in 1992–1995 to 13 cases per 100,000 in 2012–2014.
In the United States, mouth cancer and mortality rates have declined overall. However, when examined at a state level, the data reveal a more complex picture.
For instance, mouth cancer deaths have risen significantly in Nevada, North Carolina, Iowa, Ohio, Maine, Idaho, North Dakota, and Wyoming.
Some known risk factors for mouth cancer include smoking tobacco, drinking alcohol, human papillomavirus (HPV), and chewing betel quid, which is a mix of natural ingredients wrapped in a betel leaf that is popular in some parts of Southeast Asia.
New risk factor for mouth cancer
Although scientists have confirmed some risk factors, there is still much to learn about how and why mouth cancer affects certain individuals and not others. Recently, scientists set out to investigate another potential risk factor: air pollution.
The researchers, funded by the Ministry of Science and Technology in Taiwan, published their findings this week in the Journal of Investigative Medicine.
In particular, the team focused on the impact of fine particulate matter, also known as PM2.5. These are particles of liquid or solid matter that measure 2.5 micrometers in diameter or under.
Scientists already knew that PM2.5 has a negative impact on cardiovascular and respiratory health, but they wanted to find out whether exposure to higher levels of PM2.5 might also increase mouth cancer risk.
To investigate, they collated information from 482,659 men aged 40 years old or above. All participants had attended health services and given information about smoking and chewing betel quid.
The scientists next gathered data from 66 air quality-monitoring stations across Taiwan. By referring to the participants' health records, the scientists could estimate each person's exposure to PM2.5.
Risk increased by 43 percent
The researchers collected the data in 2012–2013. During this time, 1,617 men developed mouth cancer. As expected, both tobacco smoking and chewing betel quid increased mouth cancer risk.
After taking a range of influencing factors into account, the scientists demonstrated that exposure to PM2.5 also increased mouth cancer risk.
The scientists compared PM2.5 levels of below 26.74 micrograms per cubic meter (ug/m3) with those above 40.37 ug/m3. They associated the higher levels of PM2.5 with a 43 percent increase in the risk of developing mouth cancer. According to the authors:
"This study, with a large sample size, is the first to associate mouth cancer with PM2.5. [...] These findings add to the growing evidence on the adverse effects of PM2.5 on human health."
Alongside PM2.5's relationship with mouth cancer, the authors identified a correlation between higher levels of ozone and an increased risk of developing the disease.
The next challenge will be to understand how particulate matter might cause mouth cancer. Although this will require more detailed studies, some theorize that carcinogenic compounds found in PM2.5, including polycyclic aromatic hydrocarbons and heavy metals, might be part of the answer.
Because these particles have such a small diameter, the body absorbs them relatively easily, potentially causing damage as they travel through the body.
However, the authors also remind us to be cautious — this is an observational study, so it cannot definitively prove that pollution causes mouth cancer. Also, it is not clear exactly how much PM2.5 enters the mouth.
This interaction needs further investigation, but the large size of the current study makes their conclusions worthy of follow-up.