The chemicals known as PFAS are a concern nationwide. Research on health effects continues. (Photo: Colorado Department of Public Health and Environment)

Cambridge switched to using Massachusetts Water Resources Authority water Aug. 30 until the end of this year because of worries about rising levels of a group of long-lived chemicals in the city’s own drinking water, but a deeper look into the science behind measuring the risks of the chemicals shows a great deal of nuance and uncertainty. Regulators who have suggested that the concentration of the substances should be essentially zero have acknowledged gaps in data and understanding.

A City Council committee will hold a hearing next month on the chemicals, known as per- and polyfluoroalkyl substances, along with the pros and cons of using water from the regional system permanently. PFAS has been linked to harmful effects on the liver, heart, growth and immune system, among other health problems, but the strength of the evidence varies and not all people are equally at risk. The chemicals have been used in a wide variety of applications including non-stick cookware, firefighting foam, textile protectants and food wraps.

They have been found in air, dust, water, soil, breast milk and human blood, and are known as “forever chemicals” because they remain in the environment for years. Most exposure to people comes from food and drinking water.

The state now says public drinking water shouldn’t contain more than a combined 20 parts per trillion of six PFAS chemicals, averaged over three months. Federal regulators have suggested, but not set, limits as low as 0.02 parts per trillion for one PFAS chemical. That’s so low it can’t be measured using existing laboratory methods.

A 993-page analysis by a Centers for Disease Control and Prevention toxicology unit, based on approximately 600 studies, found correlations between PFAS exposure and health effects but most were in laboratory animals. Some studies of humans – workers at plants that produced or handled PFAS, people living near production facilities and the general population – found health effects but lacked specific exposure information and did not follow people over time. Scientists also haven’t discovered the cause of any health effects.

“It’s a little like having a smoking gun,” said city environmental health director Sam Lipson in an interview. The uncertainty from scientific gaps was plugged into a formula that led to the tiny limits suggested by federal regulators, Lipson said. “Communication of what that [Environmental Protection Agency] advisory standard really means is really hard,” he said. “With the public at large it’s confusing.”

Meanwhile, some residents were alarmed at the possibility that city water exceeded the much higher state limit by less than 2 parts per trillion last month, according to city councillors. How dangerous is that, equivalent to two drops in 20 Olympic-size swimming pools? The quantities are so tiny that “it’s frustrating for water systems to manage,” Lipson said.

Add to that the fact that regulators set limits based on the assumption that a person would be exposed to that amount for a lifetime, leading to different risks from the same standard based on age, he said. And risks to “sensitive” communities, such as children and people with impaired immunity, might be greater.

The state’s 20-parts-per-trillion limit looked conservative when the Environmental Protection Agency said PFAS shouldn’t exceed 70 parts per trillion in 2016. Based on more recent research, the federal agency expects to propose a new limit by the end of this year and will adopt one in 2023. In June, it issued a health advisory saying scientific studies pointed to the next-to-zero limits.

The state is examining the agency’s health advisory. The MWRA’s water, from the Quabbin Reservoir, contained no PFAS or levels below detection in two quarters last year, and the agency no longer has to report the amount. Meanwhile, the city continues to run its water treatment plant at a reduced level because “we need to keep water running through the plant” to maintain its condition, a water department official told members of the Water Board Sept. 13.

The city is also testing for PFAS in the untreated source water from its reservoirs and in treated water it no longer distributes, to find out how well filters are removing the chemicals. The department has bought new filter material and still expects to complete installation by the end of the year, water department managing director Sam Corda said. The new materials would cut levels of the chemicals in half, still much higher than the EPA’s advisory level.

In the broader picture, eight major manufacturers of PFAS started phasing out their production in the early 2000s after concerns about the chemicals mounted, leading to lower levels in blood in the general population. Scientists are studying whether replacement products pose health risks.

Future impacts of the chemicals are unknown. Research on health effects continues, including a new study in two Massachusetts communities – Ayer and Hyannis – where residents were exposed to higher levels from firefighting training sites, the CDC says. Silent Spring Institute and the Harvard T.H. Chan School of Public Health began enrolling residents of Hyannis in January and will start the study in Ayer this fall, the centers said.

The studies will measure blood levels in adults and children and will evaluate health effects.