Researchers at the Icahn School of Medicine at Mount Sinai analyzed data from 18,734 responders who live in New York City and are enrolled in one of the World Trade Center Health Clinical Centers of Excellence.
Approximately 39% of responders met criteria for probable PTSD and 35% for probable depression, according to their study, published Thursday in the journal Nature Communications.
The study is the first to examine how later-life environmental exposures may interact with World Trade Center-related chemical and psychological exposures to affect aging and mental health outcomes, according to Mount Sinai Health System.
"We know that responders who took part in the mission after 9/11 suffer health consequences to this day," Maayan Yitshak-Sade, associate professor of environmental medicine at the Icahn School of Medicine at Mount Sinai, told ABC News. "We see higher rates of PTSD and mental health conditions, and higher rates of frailty despite the relatively young age of this population. "
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Higher levels of air pollution and heat were associated with increased risks of depression, PTSD and frailty.
For each 2.48 micrograms per cubic meter increase in annual PM2.5 exposure, responders had, on average, about 2.3% higher frailty scores, 2.1% higher PTSD symptom scores and 3.9% higher depression symptom scores.
"This suggests that this initial extreme environmental exposure or stress exposure ... had a long-lasting impact, which then made them more vulnerable later in life [to] environmental exposures," Yitshak-Sade said.
Neighborhood greenness showed the opposite pattern. For each 6.67-point increase in the study's Green View Index, which measures the amount of visible tree canopy surrounding a person's residence, responders had about 1.4% lower frailty scores, 1.9% lower PTSD symptom scores and 3.8% lower depression symptom scores.
Researchers say there are several possible explanations for why greener surroundings may benefit health, including providing opportunities for physical activity and social interaction.
"Some theories suggest that even being exposed to nature ... has this calming effect [on] stress levels," Yitshak-Sade said. "So, there is no one answer that we know that is the correct answer, but we do see that residential greenness has beneficial effects on reducing mortality rates, reducing the risks of cardiovascular diseases, metabolic diseases like diabetes, and of course mental health conditions and frailty. "
Greater neighborhood greenness, however, did not eliminate the harmful health impacts researchers observed with heat and air pollution.
"Even if you do live in a green environment, we still see harmful effect of heat and air pollution," Yitshak-Sade said.
Of all the environmental factors studied, temperature appeared to have the strongest association with responders' health outcomes, accounting for 98% of the overall association with frailty and 73% of the association with PTSD symptoms.
"Temperature indeed emerges as the primary driver of the exposure mixture associations that we saw," Yitshak-Sade said. "We think it likely reflects the persistent physiological burden on the body that is ... harder for vulnerable populations to mitigate. "
The findings suggest that two people living in the same neighborhood and experiencing similar levels of heat or air pollution may not necessarily be affected in the same way, Yitshak-Sade said.
People with a history of extreme environmental or stress exposures "may be more vulnerable" to everyday environmental conditions later in life, she said, adding that the study's findings underscore the importance of continuing to monitor the health of WTC responders decades after 9/11.