The health hazards of brominated flame retardants lurking in household dust have made the news for years because of the ease with which humans can be exposed to the endocrine-disrupting compounds. Studies have hinted there are also large concentrations of unknown and potentially harmful brominated compounds in dust, but the number of compounds remained unclear and scientists didn’t have an efficient way to identify them, says Jianxian Sun, an environmental toxicologist at the University of Saskatchewan.
Now, Sun, John P. Giesy, and colleagues show that flame retardants in dust are dwarfed by another class of brominated compounds, azo dyes, that are known mutagens and commonly used to color clothing and furniture (Environ. Sci. Technol. 2016, DOI: 10.1021/acs.est.6b03954). A novel method employed by the study can sift through the thousands of compounds in complex mixtures such as dust to identify and prioritize compounds for hazard assessment.
Brominated flame retardants have received scrutiny because they disrupt hormones and impair the central nervous system. “Since dust is one of the primary routes of exposure to brominated flame retardants, we wondered if there were other brominated compounds in dust that might be of concern,” Sun says.
The team decided to take advantage of a novel screening method developed by co-author Hui Peng. Based on ultra-high resolution mass spectrometry combined with statistical analysis, the method can pluck individual brominated compounds out of a mixture and provide a chemical formula for them.
The team vacuumed up 23 dust samples from eight homes in Saskatoon, Saskatchewan, and ran them through the screening method. The researchers singled out 549 unique brominated compounds in the house dust but could find matches for only 14% of the substances in the public database Chemspider. “This study demonstrated for the first time that hundreds of novel brominated compounds exist in house dust,” Sun says.
The scientists then focused on 140 of the most abundant ones. Out of those, the statistical analysis highlighted 24 as known brominated flame retardants. The analysis also uncovered a cluster of 78 unknown, nitrogen-rich brominated chemicals that accounted for about 85% of the mass of brominated compounds in the dust. Closer investigation showed that one of the more common motifs of the unknown molecules was 2-bromo-4,6-dinitroaniline (BNA), a raw material for the synthesis of brominated azo dyes. Based this finding and the compounds’ chemical formulas, Sun and her colleagues concluded that the unknown compounds were indeed these dyes.
To pinpoint a possible source of the dyes, the team snipped samples of shirts and pants, and ran extracts of the cloth through the screening method. Because the clothes contained high concentrations of BNA and other brominated dyes, and since the dust they collected also contained clothing fibers, the scientists suggest that brominated azo dyes from fabric may be the dominant source of brominated compounds in house dust.
Finally, the researchers used a standard, cell-based test to measure the mutagenic activity of house dust and pure BNA at a concentration of 1.25 µg/mL, comparable to levels of BNA in the dust. All samples showed significant mutagenicity.
Azo dyes, most of which are nonbrominated, represent 65% of the global dye market. They are relatively cheap compounds that impart bright yellow, orange, red, and blue colors to clothing, leather, toys, and plastics. Incorporating bromine in azo dyes could make the compounds more persistent and stable in the environment, Sun says. Some of the azo dyes, with and without bromine, are known to be carcinogenic or mutagenic.
“The results are surprising because few researchers have looked at brominated azo dyes and it raises questions about why they haven’t been discovered before,” says Miriam L. Diamond, an environmental chemist at the University of Toronto. Yet the findings line up with growing recognition that textiles could be an important source of humanmade contaminants, she says. The study indicates that the research focus on flame retardants likely underestimates human exposure to brominated compounds, which are of special concern owing to their health impacts and ability to persist in the environment and bioaccumulate, Diamond concludes.