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A new study has found that exposure to polycyclic aromatic hydrocarbons significantly raises the risk of developing rheumatoid arthritis. Hinterhaus Productions/Getty Images
  • Rheumatoid arthritis is a chronic autoimmune disease that causes pain and swelling in the joints and may lead to other symptoms.
  • Symptoms can be managed with medications, physical therapy, occupational therapy, counseling, and surgery.
  • Rheumatoid arthritis is more common in women, people over 60, and those who have a close relative with the condition.
  • The exact causes of Rheumatoid arthritis are not known, but certain factors may increase risk.
  • A new study has found that exposure to polycyclic aromatic hydrocarbons — chemicals formed when certain substances are burnt — may significantly increase the risk of developing rheumatoid arthritis.

Rheumatoid arthritis (RA), a chronic and progressive autoimmune disease, affects up to 1% of people globally. In the United States, around 1.3 million adults live with the condition.

Rheumatoid arthritis causes a range of symptoms, the most common being inflammation, pain, and swelling in the joints, often starting in the hands and feet. As an autoimmune disorder, symptoms are caused by a person’s immune system attacking healthy tissues, mistaking them for harmful pathogens.

The more common form of arthritis, osteoarthritis, which results from wear and tear on the joints, is not an autoimmune disorder.

Several factors increase the risk of developing rheumatoid arthritis. According to the Centers for Disease Control and Prevention (CDC), these include:

  • increasing age, with onset being most likely over the age of 60
  • females are 2 to 3 times more likely to develop RA than men
  • genetics or inherited traits
  • tobacco use, which can both increase the risk of developing RA and make the disease worse
  • females who have never given birth
  • individuals with parents who smoked
  • some early life exposures
  • people who have overweight or obesity

The prevalence of rheumatoid arthritis is greater in industrialized countries, which scientists say may be partly explained by environmental factors.

A new study has highlighted the importance of one particular environmental factor — exposure to polycyclic aromatic hydrocarbons (PAH), chemicals found in coal, crude oils, gasoline, and tobacco.

The observational study found that the risk of rheumatoid arthritis was highest among individuals with the highest levels of environmental exposure to PAH chemicals, regardless of their smoking status.

The results were recently published in BMJ Open.

Polycyclic aromatic hydrocarbons are chemicals that are released when coal, oil, gasoline, wood, tobacco, and garbage are burnt, and when meat and some other foods are burnt during cooking.

According to the CDC, people are exposed to PAHs when they:

  • breathe air containing motor vehicle exhaust, cigarette or wood smoke, or fumes from asphalt roads
  • eat grilled or charred meats or foods
  • eat foods on which PAH particles have settled from the air
  • experience skin exposure to PAH

Exposure to PAHs has been linked with several health conditions, including some cancers, asthma, cardiovascular disease, and obstructive lung diseases, according to the World Health Organization (WHO).

To assess the association between the exposure to certain environmental toxicants and rheumatoid arthritis, the researchers drew on data from the U.S. National Health and Nutrition Examination Survey (NHANES) between 2007 and 2016.

The NHANES study included a total of 21,987 adults, 1,418 of whom had rheumatoid arthritis and 20,569 of whom did not. During the NHANES study, blood and urine samples were collected to measure the total amount of different toxicants in the body.

For the new study, the researchers looked for connections between levels of different toxicants and the incidence of rheumatoid arthritis. They controlled for other factors that might influence the risk of developing the condition, including dietary fiber intake, physical activity, smoking, household income, educational attainment, age, sex, and body weight.

The researchers found that those in the top 25% of bodily PAH levels had the highest likelihood of developing rheumatoid arthritis, whether or not they were, or had ever been, smokers.

Only one chemical — PAH 1-hydroxynaphthalene, which is found in many household and personal care products — was strongly associated with an increased rheumatoid arthritis risk.

Smoking is a known risk factor for rheumatoid arthritis.

In this study, the researchers found that 90% of the increased rheumatoid arthritis risk from smoking was due to PAH exposure.

Dr. Dung Trinh, brain health expert and Chief Medical Officer of The Healthy Brain Clinic in Long Beach, CA, told Medical News Today:

“While previous research has focused on the association between smoking and rheumatoid arthritis risk, this study highlights the significant contribution of PAH exposure to the relationship between smoking and the disease. It suggests that PAHs not only account for most of smoking’s impact on rheumatoid arthritis risk but also independently contribute to the development of the disease.”

The current study was observational, which means there are limitations to the findings.

“It’s important to consider that observational studies can only demonstrate associations or correlations, not causation,” Dr. Trinh noted.

“Therefore, further research, such as controlled experiments or longitudinal studies, would be needed to establish a more definitive link between PAH exposure and rheumatoid arthritis risk.”

The researchers acknowledge the limitations of their findings but say the study adds to evidence that environmental PAHs are linked to rheumatoid arthritis, regardless of smoking status. They called for further research into the relationship between socioeconomic status, PAHs, and rheumatoid arthritis.

“To our knowledge, this is the first study to demonstrate that environmental exposure to PAHs mediate the majority of the association between smoking and rheumatoid arthritis, and also contribute to population burden of rheumatoid arthritis independently of smoking status,” the study authors wrote.

There are several treatments that can help people with Rheumatoid arthritis, but there are also lifestyle changes that can help alleviate symptoms.

Avoiding smoking is one way to lower your risk of rheumatoid arthritis. Not only has this study and others shown that smoking increases the risk of developing the condition, it may also exacerbate symptoms.

Resting, particularly during rheumatoid arthritis flares may help alleviate the pain.

According to Dr. Trinh, people can help to reduce symptoms of rheumatoid arthritis if they:

  • maintain a healthy weight to reduce stress on joints
  • engage in low-impact exercises like swimming or cycling
  • practice gentle stretching to improve flexibility

But one factor that is difficult to modify is low socioeconomic status. As the study authors point out:

“…households of lower socioeconomic status generally experience poorer indoor air quality and may reside in urban areas next to major roadways or in high traffic areas.”

This, they suggest, might make these individuals particularly vulnerable to rheumatoid arthritis.

And studies have shown that people with lower socioeconomic status do have a greater incidence of rheumatoid arthritis. Improving the living conditions of lower-income households could be an important step in reducing the impact of this chronic disorder.