Exposure to extreme heat could lead to faster aging, a new study published today in the journal Science Advances suggests. Older people living in hotter areas of the US showed faster aging at the molecular level than people living in cooler areas.

The study looked at measures of a person’s biological, or epigenetic, age, which is based on how a person’s body is functioning at the molecular and cellular levels and doesn’t necessarily match a person’s chronological age based on birth. Longer-term exposure to heat was associated with an increase in a person’s biological age by up to 2.48 years. The impact on the body is comparable to the effects of smoking, according to the study authors.

“We’re kind of surprised [at] how massive this impact could be,” says Eun Young Choi, lead author of the study and a postdoctoral associate at the University of Southern California. “The effects of extreme heat might not show up right away as a diagnosable health condition, but it could be taking a silent toll at the cellular and the molecular level which could years later develop into disability and disease.“

The research included blood samples collected from 3,686 adults aged 56 or older living across the US. The study authors compared those samples with heat index data, a measure of temperature and humidity, between 2010 and 2016. They found a correlation between greater exposure to extreme heat and a bigger jump in epigenetic age. A person living in a place where the heat index is 90 degrees Fahrenheit or above for half the year experienced up to 14 more months of biological aging compared to someone living somewhere with less than 10 days a year that hot.

“The thing that is interesting here is that a lot of observational data focuses on acute impacts of extreme heat exposure – this paper underlines that there may be chronic impacts on epigenetic age that are important predictors of adverse health,” Amruta Nori-Sarma, deputy director of Harvard Chan C-CHANGE and assistant professor of environmental health and population sciences says in an email to The Verge.

Nori-Sarma and Choi say it’s important to keep in mind, however, that the study doesn’t take into consideration whether a person had access to air conditioning or other ways to stay cool. There’s room for more research into what factors might make an individual more resilient or more vulnerable to heat.

“Our finding doesn’t necessarily mean that every person living in Phoenix, Arizona, for example, has an older biological age. This is really an average impact,” Choi says. “Two people in the same neighborhood could have very different levels of personal exposure depending on whether they have air conditioning.”

That also shows that there are steps that can be taken to keep people safe in a warming world. Aside from stopping climate change, that can look like planting more trees and painting rooftops white to prevent urban areas from trapping as much heat, and opening up more public spaces where people can get access to air conditioning. Finding solutions gets easier to do when people are more aware of the potential risks.