A Parkinson's “timebomb” has sparked fears thousands could develop the disease after being exposed to a once commonly used chemical.
The toxic chemical was used in dry cleaning and new research has warned it could actually cause Parkinson’s.
Scientists have said that people exposed to the industrial solvent are 70 per cent more likely to develop the disease.
The findings are based on tens of thousands of former marines stationed at Camp Lejeune, in North Carolina.
Its water supply was contaminated with trichloroethylene (TCE) and other volatile organic compounds.
Corresponding author Professor Samuel Goldman, of California University in San Francisco, said: "Exposure to trichloroethylene in water may increase the risk of Parkinson's disease.
"Millions worldwide have been and continue to be exposed to this ubiquitous environmental contaminant."
It was also found in decaffeinated coffee.
TCE has been banned by the food and pharmaceutical industries since the 1970s.
It was removed from dry cleaning in the mid 1950s but is still used in metal cleaning and degreasing and as an extraction solvent in the textile manufacturing industry.
Camp Lejeune drinking water was contaminated with TCE from 1953 until 1987 when testing uncovered the polluted wells.
Prof Goldman said: "Monthly median levels of TCE in the base's water supply were greater than 70-fold the permissible amount."
His team compared the number of Parkinson's cases among around 172,000 veterans from Camp Lejeune and 168,000 peers at Camp Pendleton in California - where drinking water was clean.
They were mostly male and spent an average of about two years living on their respective bases between 1975 and 1985, when the contamination was at its highest.
More than three decades later, 279 veterans from Camp Lejeune and 151 from Camp Pendleton had been diagnosed with Parkinson disease - for a prevalence of 0.33 and 0.21 percent respectively.
Prof Golman said: "Computer models showed Camp Lejeune veterans had a 70 percent higher risk of Parkinson's than Camp Pendleton veterans."
Up to a third of water supplies in the US have measurable amounts of TCE - leading to accumulation in soil.
It can be absorbed by humans through the lungs, skin and intestines.
Exposures may occur through occupational usage, ingestion of contaminated food and water and during cooking and bathing.
Prof Goldman said: "Reflecting its environmental ubiquitousness, TCE has been broadly detected in human breast milk, blood and urine.
"It should be noted that in addition to the exposed service members studied here, hundreds of thousands of family members and civilian workers exposed to contaminated water at Camp Lejeune may also be at increased risk of Parkinson's, cancers and other health consequences.
"Continued prospective follow-up of this population is essential.
"This cohort study’s findings suggest the risk of Parkinson's is 70 per cent higher in veterans who were exposed to TCE and other VOCs 40 years ago.
"Trichloroethylene is a ubiquitous environmental contaminant used throughout the world since the 1920s. Many millions have been and continue to be exposed."
A previous global study by the same team found it increased the risk of the neurological condition sixfold.
About one million people in the US. currently suffer from the condition. Doctors diagnose 60,000 Americans each year.
The study is in JAMA Neurology.
