NEW YORK (NYTIMES) - At some point last summer, there were just too many reports of protesters who had experienced abnormal menstrual cycles after being exposed to tear gas for Britta Torgrimson-Ojerio, a nurse researcher at the Kaiser Permanente Centre for Health Research in Portland, Oregon, to dismiss them as coincidence.

A preschool teacher told Oregon Public Broadasting that if she inhaled a significant amount of gas at night, she would get her period the next morning.

Other Portland residents shared stories of periods that lasted for weeks and of unusual spotting.

Transgender men described sudden periods that defied hormones that had kept menstruation at bay for months or years.

Torgrimson-Ojerio decided she would try to figure out whether these anecdotes were outliers or representative of a more common phenomenon.

She surveyed around 2,200 adults who said they had been exposed to tear gas in Portland last summer.

In a study published this week in the journal BMC Public Health, she reported that 899 of them - more than 54 per cent of the respondents who potentially menstruate - said they had experienced abnormal menstrual cycles.

"Even though we cannot say anything scientifically definitive about these chemical agents and a causal relationship to menstrual irregularities," Torgrimson-Ojerio said, "we can definitively say that in our study most people who had menstrual cycles or a uterus reported menstrual irregularities after reporting exposure to tear gas."

Torgrimson-Ojerio and her colleagues recruited survey participants through social media and links on the websites of The Oregonian and the Oregon Health Authority in July and August.

The researchers asked participants to explain precisely how their periods had been affected after exposure to tear gas. Increased cramps, unusual spotting and uncharacteristically intense or long bleeding were the most common reactions.

A number of people who do not usually have periods because of hormone therapy or age reported unexpected bleeding and spotting, Torgrimson-Ojerio said.

This study has limitations. It is not a random sample. And given that subjects were permitted to participate anonymously, researchers could not verify their accounts.

Nor can the study answer how or why tear gas might be contributing to menstrual irregularities or to what extent other factors are also involved.