Combinations of air pollutants impact pediatric asthma symptoms

Key takeaways:

• Symptoms were associated with 2-nitropropane and 2,4,6-trichlorophenol across three exposure periods.
• Other pollutants associated with symptoms included 1,2,4-trichlorobenzene and 1,3-butadiene.

Researchers used machine learning to identify 25 combinations of air pollutants that were associated with increased asthma symptoms among children in Spokane, Washington, according to a study published in Science of the Total Environment.

Also, a vulnerable sub-population was exposed to 13 of these combinations, Solmaz Amiri, DDes, MSc, research assistant professor, Institute for Research and Education to Advance Community Health, Elson S. Floyd College of Medicine, and colleagues wrote.

“There is literature around the effect of exposure to air pollutants on asthma,” Amiri told Healio. “However, relationships are studied using a limited number of pollutants, including particulate matter, nitrogen dioxide, and ozone, among others. It is not just one or two pollutants that can be linked to asthma symptoms.”

The U.S. Environmental Protection Agency National Air Toxics Assessment, conducted prior to 2017, and the Air Toxics Screening Assessment, conducted from 2017 onward, indicated 109 air pollutants including carbonyls, heavy metals, chlorophenols and volatile organic compounds present in Spokane.

“Some of these air toxics were discontinued in the U.S., but they can still be found in materials that may be in storage or people have in their backyard or garage,” Amiri said. “Other air toxics still exist at least in the environment.”

The Data-driven ExposurE Profile (DEEP), which uses machine learning, identified combinations of these pollutants associated with asthma symptoms. It also established epidemiologic methods control for confounding as well as identified combinations and synergistic interactions with significant associations, the researchers said.

Next, the researchers analyzed these combinations against the total number of asthma symptoms experienced by 269 children with an asthma plan (mean age, 9.7 years; 51.3% girls) at 10 elementary schools in the Spokane Public Schools system between September 2019 and February 2020.

The cohort included 59% who were non-Hispanic white, 22% who were non-Hispanic multi-racial and 12% who were Hispanic, with 75% of these children eligible for free or reduced meals.

Also, 38% of these children did not experience any asthma symptoms at school during the study period, whereas those who did experienced an average of 9.6 symptoms (standard deviation, 16.9).

Pollutant exposures

DEEP identified increased levels of 14 single and 11 multiple combinations of air pollutants associated with worsening asthma symptoms in at least one of three exposure periods, defined as the most recent year (2019), the previous 3 years (2017-2019) and the previous 5 years (2014-2019). Wheezing, coughing, chest tightness, dyspnea, breathlessness and inhaler use were included in the reported symptoms.

The 14 single combinations included 2-nitropropane and 2,4,6-trichlorophenol, which were significantly associated with outcomes across all three exposure periods, suggesting their sustained impact on asthma symptoms, the researchers said.

Also, 1,2,4-trichlorobenzene, 1,3-butadiene, acetaldehyde, acetonitrile, benzene, cumene, diesel particulate matter and ethylbenzene had individual associations with asthma symptoms, the researchers continued.

The 11 multiple combinations included 10 that comprised two pollutants each and one that comprised three pollutants. Five of these combinations included 1,1,1-trichloroethane, four included 1,3-butadiene, and three included 2,4,6-trichlorophenol.

“One of the air pollutants, i.e., 1,1,1‐trichloroethane, was associated with asthma in combination with other air pollutants and would not have been identified without the use of machine learning algorithms,” Amiri said.

Specifically, 1,1,1-trichloroethane, 1,1,2,2-tetrachloroethane, BIS (2-ethylhexyl) phthalate (DEHP) and 2,4-dinitrophenol only appeared in combinations and had no individual associations with outcomes. The researchers noted that they would not have been able to identify these pollutants without DEEP.

Across all three exposure periods, 1,1,1-trichloroethane and 2,4,6-trichlorophenol were significantly associated with asthma symptoms, the researchers said.

Constituents of six of the multiple combinations of pollutants displayed significant synergistic interactions during one exposure period or more, including three combinations where 2,4,6-trichlorophenol was the primary pollutant.

Also, 1,1,1-trichloroethane was identified in two of these three combinations, and this pollutant appeared in five combinations with significant interactions, the researchers continued.

All three exposure periods included interactions where 2,4,6-trichlorophenol was present, and 1,1,1-trichloroethane was present in interactions in two exposure periods, which the researchers said indicated their sustained impact on asthma.

“We also found socioeconomic disparities,” Amiri said. “Schools with a higher percentage of students who qualify for free or reduced meals were exposed to a wide variety of air pollutants in the neighborhoods where they live.”

The researchers further found 28 children with exposure to 13 of these 25 combinations during one or more of the exposure periods. Compared with the full cohort, these children were older (9.9 years vs. 9.7 years; P < .001). Also, 91% received free or reduced cost meals, compared with 73% in the full cohort (P < .001).

These greater exposures among children from lower-income areas, indicated by the percentage of students receiving free or reduced cost meals, is consistent with previous studies, the researchers said.

Sources and implications

Also known as methyl chloroform, 1,1,1-trichloroethane is a common industrial solvent that was previously used in household cleaners, glues and aerosol sprays. The lungs retain up to 30% of it when it is inhaled, the researchers said.

Exposure has been associated with irritated nasal and respiratory mucous membranes, with shortness of breath, cough, and chest tightness, as well as with daily use of asthma medication, the researchers continued.

As a chlorophenol, 2,4,6-trichlorophenol was used as an antiseptic and anti-mildew agent in the United States and can still be found in pesticides and preservatives produced before its discontinuation in 1980.

The researchers reported that chronic exposure to 2,4,6-trichlorophenol increases risks for asthma and respiratory issues including cough, chronic bronchitis, chest wheezing, altered pulmonary function and pulmonary lesions.

The EPA has characterized 1,3-butadiene, benzene, acetaldehyde and 1,1,2,2-tetrachloroethane, which were present in combinations identified by DEEP, as serious threats to human health in urban areas.

Sources include vehicles and vehicle service stations, manufacturing, forest fires, tobacco smoke, rubber and plastic production, and burning coal and oil. Specifically, 1,3-butadiene has been associated with irritation in the respiratory tract, and benzene has been associated with mucous membrane irritation, acute granular tracheitis, laryngitis, bronchitis and pulmonary edema.

Acetaldehyde irritates the respiratory tract and triggers cough and pulmonary edema, leading to bronchoconstriction among patients who have asthma, the researchers said. It is a common chemical intermediate in perfumes, polyester resins, and dyes. Also, it is a solvent used in rubber, tanning and paper manufacture. It has been associated with ED visits related to asthma among children as well, the researchers said.

Considering the amount of time that children spend in school, the researchers emphasized the importance of monitoring and reducing concentrations of air pollutants around schools, with additional efforts in identifying vulnerable school sites.

The researchers also called for additional studies investigating the extent of the associations between environmental and socioeconomic characteristics and asthma symptoms.

“Our findings suggest that health outcomes associated with environmental exposures should be studied by obtaining multi-dimensional exposure data, especially in settings with vulnerable populations,” Amiri said.

Multi-dimensional exposure data, particularly from settings where vulnerable populations reside, would benefit these studies of adverse environmental health outcomes including worsening asthma symptoms, the researchers continued.

Finally, the researchers said, these data would be useful in drafting regulations that target these pollutants.

“Future research will examine the extent to which exposure to air toxics affects children’s overall and respiratory health, along with the biological mechanisms that drive those effects,” Amiri said.

Reference:

• Multiple air pollutants linked to asthma symptoms in children. https://news.wsu.edu/press-release/2024/03/13/multiple-air-pollutants-linked-to-asthma-symptoms-in-children/. Published March 13, 2024. Accessed March 25, 2024.