Chemicals found in common household disinfectants, glues, and furniture textiles could damage supporting cells in the brain during critical stages of their development, a new study based on human cell cultures and mice has found.
Starting with 1,823 compounds of unknown toxicity found in the environment, Erin Cohn, a molecular biologist at Case Western Reserve University in Ohio, and colleagues identified two classes of chemicals that either killed or halted the maturation of cells called oligodendrocytes under laboratory conditions.
Oligodendrocytes are a type of neurological support cell: They wrap around neurons to form an insulating covering that keeps brain signals scooting along at speed.
One of the two chemical classes identified were quaternary compounds. These are used in some disinfectant sprays, wipes and hand sanitizers, and personal care products such as toothpaste and mouthwash to kill bacteria and viruses, and can be ingested or inhaled if used incorrectly or in poorly ventilated spaces.
The other class of compounds were organophosphates. Serving as flame retardants, they are commonly found in textiles, glues, and household items such as furniture and electronics and can 'off-gas' into the air of rooms we commonly spend time in. Being fat-soluble, organophosphates can be absorbed through the skin and potentially make their way into the brain.
In mice experiments, pups given an oral dose of one of three quaternary compounds had detectable levels of those chemicals in their brain tissue days later, suggesting that the compounds can cross the blood-brain barrier, the protective fortress between the bloodstream and brain cells.
The animals also had depleted numbers of oligodendrocytes in their brains after 10 daily doses of one particular quaternary compound, cetylpyridinium chloride, administered in a key period of brain development, starting five days after birth.
Similar effects were seen in brain organoids; clusters of human stem cells grown in a dish and coaxed into behaving like developing brain tissue.
"We found that oligodendrocytes – but not other brain cells – are surprisingly vulnerable to quaternary ammonium compounds and organophosphate flame retardants," says Cohn.
Experts, however, aren't immediately alarmed by the results of this lab-based study, which treated cells and mice with concentrations higher than what humans would typically be exposed to, and in ways that don't reflect how humans would come into contact with these chemicals.
"It's not a question of if something is toxic or not but if it is toxic under the conditions to which we are likely to be exposed," says environmental chemist Oliver Jones of RMIT University in Melbourne, Australia.
"In this case, the authors have exposed cells in a Petri dish to a relatively high amount of these compounds which is not the same dose route or duration of exposure that humans might encounter normally."
However, we know that some people, such as school and hospital cleaners, childcare providers, and people in correctional facilities, are more exposed to quaternary compounds than others because industrial-strength disinfectants are commonly used in these settings.
What's more, until only recently, research into the toxicity of quaternary compounds came from animal and cell studies.
But now studies are probing the health effects in humans, and finding that people had double the levels of quaternary compounds in their blood during the pandemic than before – likely because of the widespread use of disinfectants.
This has led some researchers to flag these chemicals as a "class of emerging concern", noting the challenges of fully understanding the human health risks of these compounds due to their vast structural diversity.
It's a similar situation with organophosphate flame retardants, which have been widely detected in the environment, and in human blood, urine, plancental tissue, and breast milk.
"Watch out for them to come to the attention of the Stockholm Convention when it's done struggling with organofluorines (PFAS)!" says environmental chemist Ian Rae, who is an advisor to the United Nations Environment Programme on chemicals in the environment and who was not involved in the current research.
In their study, Cohn and colleagues analyzed levels of one flame retardant by analyzing levels of one flame retardant metabolite, BDCIPP, in children's urine samples collected in the US CDC's National Health and Nutrition Examination Survey between 2013 and 2018.
Almost all of the 1,763 children aged 3 to 11 years had BDCIPP present in their pee. Those with the highest levels were two and six times more likely than those with low exposure to experience adverse neurodevelopmental outcomes such as motor dysfunction or requirements for educational assistance.
But observational data can only point to associations, not direct causes, and large gaps remain in our understanding of how these chemicals affect humans since like this study, most of the data comes from animals and cells.
That's reason enough to keep investigating the health effects of these compounds, especially in children, Cohn and colleagues argue: "The developing central nervous system is particularly sensitive to environmental insults, and chemical exposures can be especially harmful to children if they occur during critical periods of development."
The study has been published in Nature Neuroscience.
Editor's update (Thursday 11 April 2024): A previous headline for this article suggested hand sanitizers could damage brain cells. The headline has been changed to more accurately reflect the research finding.