Human milk reflects not only infant nutrition, but also the environments in which parents live. A new set of studies suggests that those environments may be more chemically complex than previously understood.
An interdisciplinary research team that includes scientists from McGill University identified a range of unexpected chemical compounds in human milk samples collected in Canada and South Africa. The findings, published across five peer-reviewed papers, highlight how widespread environmental exposures can appear in biological samples, even when health effects are not yet clear.
The detected compounds included traces of pesticides, antimicrobial preservatives and additives used in plastics and personal-care products. Many were found at low concentrations, and researchers stress that the presence of these substances does not change existing guidance on infant feeding.
“It is important to note that these chemicals were detected at low concentrations, and we do not fully understand the health effects of many of them,” said Stéphane Bayen, associate professor in McGill’s Department of Food Science and Agricultural Chemistry and a co-author on the studies. “So, despite these findings, breast milk remains ideal for infants, as it has the nutrients infants need to develop as well as antibodies that protect them against diseases.”
Unlike earlier research that typically targeted specific chemicals, the team used a non-targeted analytical approach designed to scan broadly for known and unknown compounds. Using high-resolution mass spectrometry, researchers analyzed 594 human milk samples collected between 2018 and 2019 from Montreal and from regions in South Africa including Vhembe and Pretoria.
This approach revealed several compounds not previously reported in human milk, including antimicrobial preservatives commonly added to soaps and disinfectants, plastic-related antioxidants used in manufacturing and certain pesticide-related chemicals. In samples from South Africa, the researchers also detected 8-hydroxyefavirenz, a breakdown product of efavirenz, a medication used to treat HIV that was widely prescribed during the sample years.
“To our knowledge, the compound had never before been identified in human milk,” Bayen said. Its presence, he added, likely reflects medication use during or prior to the study period rather than ongoing exposure.
The researchers emphasize that detection alone does not indicate harm. Establishing baseline data, they say, is a necessary step toward understanding how complex mixtures of environmental chemicals move through the body and where safety testing may need to expand.
“Our results highlight — perhaps unsurprisingly — that populations are exposed to a complex cocktail of chemical residues, reflecting each individual’s diet, environment and lifestyle,” Bayen said.
One of the studies did find associations between concentrations of certain bisphenols in breast milk and altered growth patterns among South African infants. However, the authors caution that this was the first study of its kind and that findings need to be replicated before drawing conclusions.
“This is the first study of this type, and so results need to be replicated before any conclusion can be drawn,” said Jonathan Chevrier, associate professor of epidemiology and a co-author of the research.
Chevrier emphasized that understanding exposure during early life remains important precisely because human milk is so central to infant development.
“Human milk is considered the gold standard for infant nutrition,” he said. “It is therefore essential to understand everything infants are exposed to during this critical window of susceptibility in development.”
The researchers say the findings should not be interpreted as a reason to alter breastfeeding practices. Instead, they point to broader implications for chemical safety regulation and the need for improved testing frameworks that reflect real-world exposure patterns.
The research was funded by the Canada Foundation for Innovation, the Canadian Institutes of Health Research and a Canada Research Chair in Global Environmental Health and Epidemiology.