New study sees link between pesticide and arthritis

By Grace van Deelen

Exposure to a commonly used pesticide could put people at higher risk of developing rheumatoid arthritis, according to a new study.

The findings, published last week in the journal Environmental Science and Pollution Research, looked at potential links between the disease and a class of chemicals known as pyrethroids, which are found in many commercial products used to control insects, including household bug killers, pet sprays and shampoos.

Using data collected as part of a long-term study of US residents called the National Health and Nutrition Examination Survey, the research team said they looked at levels of pyrethroids found in urine samples of the more than 4,000 study participants and analyzed how the exposure correlated with incidences of the disease.

The research team, which was led by scientists from Anhui Medical University in Hefei, China, concluded that levels of pyrethroid indicators in the urine of those who had self-reported a rheumatoid arthritis diagnosis were “significantly higher” than those who had not reported the diagnosis.

More research needed

The work adds to years of scientific research showing that many pesticides are associated with a range of adverse human health impacts, including cancer, neurodevelopmental harm and reproductive problems.

Prior research has also indicated that pesticide use may play a role in the development of rheumatoid arthritis, an autoimmune disease that attacks the joints and can cause pain, bone erosion, and joint deformities. But little work has been done regarding possible associations between specific pesticides and the disease, the researchers said.

The authors write that there is still a “great need” for other research to replicate their findings.

Pyrethroid pesticides are often used to kill mosquitos in outdoor recreational areas and for residential use under brand names that include Baygon, Anvil, and Scourge. People can absorb the pesticide through inhalation, ingestion, or skin contact.

The EPA has acknowledged that the pesticides are toxic to organisms other than insects, including mammals, though the agency has continued to approve new products containing pyrethroids.

The market for pyrethroid pesticides is growing as pyrethroids are replacing other pesticides considered more dangerous. As a result, pyrethroid pesticides are some of the most commonly used globally, with one 2020 study finding that they comprise more than 30 percent of the global pesticide market.

Study draws skeptics

The study has drawn some criticism from other scientists who say the conclusions are too strong given unclear findings.

“You really cannot infer causality from this type of study,” said Carly Hyland, a postdoc at Boise State University who studies the health impacts of pesticides.

Hyland said the methods used in the study appear insufficient to determine an association. She said measurements of pyrethroid indicators in urine reflect the most recent exposure of a person to pyrethroid – perhaps no more than 24 hours worth of exposure, while rheumatoid arthritis develops over many years.

Anneclaire De Roos, an associate professor of environmental health at Drexel University who has published research linking pesticide exposure and rheumatoid arthritis, agreed.  She said subsequent research should measure chronic pyrethroid exposure rather than recent exposure.

“It’s definitely worth studying again,” she said.

Nathan Donley, the environmental health science director at the Center for Biological Diversity, said there’s still cause for concern despite the study’s shortcomings. “It’s important to do these sorts of studies, because it gives us a sense of what’s going on,” he said. “When a pesticide gets approved, there’s this assumption that we’ve looked at everything, and we found that it’s safe. But inevitably, every single pesticide is found to be more harmful than it was originally estimated.”