Exposure to low levels of hydrogen sulfide and possibly other airborne chemicals from nearby industrial plants may have contributed to a significantly increased suicide rate in one community, according to a presentation at the 17th Annual U.S. Psychiatric and Mental Health Congress.

In 2003, the suicide rate in two neighborhoods of a single American town was found to be 192 per 100,000 individuals a year, roughly 16 times the statewide average. A total of 1,561 people lived immediately downwind from a liquid asphalt terminal, an asphalt hot-mix plant, and a site known to be compromised by chemical contaminants in the soil. Retrospective analysis of death certificates showed that between 1994 and 2003, the two neighborhoods had a 3.5-fold statistically significant increase in the suicide rate.

Lead author and presenter Richard H. Weisler, MD, said “For example, here in the block group 1 neighborhood in the mid-90s, we found 1 death by suicide for about every 230 people during the worst 12-month period, versus an average of 1 death by suicide for every 8,621 people in the rest of North Carolina. When we saw this data it gave us pause.” Weisler added that the odor of hydrogen sulfide had been apparent for decades to residents and visitors.

In 2001, scientists estimated the average maximum hydrogen sulfide level in a large part of the affected area was 215 parts per billion (ppb), while some sections of the neighborhoods were reported as low as 30 ppb. Based on computer modeling, they estimated that historical releases of hydrogen sulfide reached average maximum levels of 860 ppb in a few residences very near the asphalt facilities. By comparison, the World Health Organization has a 10-minute exposure standard of 5 ppb.

In addition to suggestions of an increased suicide rate, the incidence rate of primary brain cancers in these neighborhoods from 1995 to 2000 showed an increase about 6.4 times greater than expected for the population, possibly due to benzene and other solvent exposures, Weisler said.

Weisler and his study team made a hypothetical link between hydrogen sulfide and suicides due to biological plausibility. They noted that hydrogen sulfide affects brain neurochemistry as a direct gaseous neuromodulator that potentially affects mood states and the psychological stress response. In animal studies, it has been shown to alter the neurotransmitters serotonin, norepinephrine, dopamine, aspartate and glutamate levels.

Hydrogen sulfide also affects the hypothalamic pituitary adrenal axis and corticotropin releasing factor in animal studies, he said.

The study team reported that additional neurotoxic compounds such as benzene, chlorinated solvents and carbon disulfide, among others, were released in unknown quantities by the asphalt terminal and hot-mix asphalt plant. Carbon disulfide, also a neurotoxin, has been linked to personality changes, mood disorders and suicides in occupational settings, the researchers said.

The confirmation of high levels of airborne chemicals including hydrogen sulfide over long periods of time places new emphasis on understanding environmental contributors to central nervous system function, including effects on mood such as vulnerability to severe depression and suicidal ideation.