Prolonged exposure to a chemical commonly used as a cleaning agent, particularly in dry cleaning clothes and degreasing metals, can increase the risk of Parkinson's disease by 70 per cent, according to a study.

Trichloroethylene (TCE) is a liquid chemical that lingers in the air, water, and soil for decades. Previous research has linked TCE, or trichloroethylene, to certain cancers, but a new study published in JAMA Neurology demonstrated its association with Parkinson's.

TCE has been used for industrial and commercial purposes for nearly 100 years and was used as a surgical anaesthetic until it was banned in 1977.

Today, it is primarily used to degrease industrial metal parts. This entails heating TCE in degreasing tanks to create a vapour that dissolves the grease, but it also releases the chemical into the atmosphere. Once TCE enters the soil or groundwater, it can persist for decades.

In the study, researchers led by the University of California-San Francisco and the San Francisco VA Medical Center compared Parkinson's diagnoses in approximately 160,000 Navy and Marine veterans.

Researchers found that 430 veterans had been diagnosed with Parkinson's and that their risk was 70 per cent higher than those not exposed.

The civilian population is also at risk of TCE exposure, said first author Samuel M. Goldman, of the UCSF Division of Occupational, Environmental and Climate Medicine, noting that between 9 per cent and 34 per cent of US water supplies contain measurable amounts of the chemical.

"TCE is still a very commonly used chemical in the United States and throughout the world. Its production has been increasing over the past several years and it is widely available online," he said.

"Unfortunately, there's no easy way to know if you've been exposed, unless you've worked with it directly. Many of us have detectable levels of TCE in our bodies, but it gets metabolised and excreted very quickly, so blood and urine tests only reflect very recent exposure."

Additionally, the researchers found that the exposed veterans had a higher prevalence of prodromal Parkinson's -- symptoms that are suggestive of Parkinson's but do not yet fulfil the diagnostic criteria for the disease.

"Loss of sense of smell, a sleep disorder known as RBD, anxiety, depression and constipation can be early signs of Parkinson's, but only a very small fraction of people with them will develop it," said Caroline M. Tanner, of the UCSF Department of Neurology.

The risk of developing Parkinson's in the future can be estimated using a risk score based on these symptoms, Tanner said.