Workers involved in mustard-gas production during the World War II era showed heightened odds of lung cancer at a relatively young age - with the excess risk fading in old age, a new study finds.

Japanese researchers found that of workers employed at a poisonous-gas factory between 1929 and 1945, those directly involved in producing mustard gas saw their risk of lung cancer, while still rare, increase earlier in life compared with other workers.

For each year of exposure to mustard gas, workers’ lung cancer risk increased three to five years earlier, the researchers report in the American Journal of Epidemiology.

However, among workers who survived into old age, those involved in producing mustard gas showed no increased rate of lung cancer.

Mustard gas is one of the oldest chemical weapons employed by military forces around the world. The poison reportedly caused 78 percent of British gas casualties in World War I. Use of mustard gas has been documented in 11 subsequent conflicts, most recently by Iraq against its own citizens and against Iran in the 1980s.

It has long been known that when mustard-gas exposure does not kill instantly, it raises victims’ cancer risk later on.

These latest findings offer more insight into the long-range cancer risk, according to Dr. Noboru Hattori, an associate professor at Hiroshima University and one of the researchers on the study.

The patterns among these workers - an increased cancer risk earlier life that fades with age - are similar to what’s been seen among Japan’s atomic-bomb survivors, Hattori told Reuters Health by email.

The study included 480 former workers at a single Japanese poisonous-gas factory who were exposed to mustard gas, 233 exposed to other chemicals, and 969 employees not directly involved in manufacturing chemicals.

The group was followed from 1952 to 2005.

Early on, mustard-gas workers showed a heightened lung cancer risk. Between 1965 and 1974, seven developed lung cancer - a rate of one in 600 per year - versus no cases among unexposed workers.

Similarly, the lung cancer rate from 1974 to 1994 was roughly one in 250 per year among mustard-gas workers - versus one in 2,000 to one in 1,000 per year among workers with no chemical exposures.

However, for the years 1995 to 2005, lung cancer rates were similar in both groups, at about one in 200 per year.

Since the study looked at a specific group with work exposure to chemical agents, the findings cannot be directly applied to other people who may have been exposed to mustard gas, Hattori said.

However, the researchers write in the report, chronic lung damage has also been reported among people exposed to mustard gas during the Iran-Iraq conflict of the 1980s.

“Unfortunately,” Hattori said, “there are no ways to lessen the long-term risks of mustard-gas exposure.”

SOURCE: American Journal of Epidemiology, online February 18, 2011.