New research linked air pollution to more than 30,000 U.S. deaths in 2015.

The study was released on Tuesday.

It found that even when air levels are considered to be safe, death rates can increase.

While air quality has increased since 1999, researchers say the findings suggest even stricter regulations are needed.

The study looked for links between pollution and cardiovascular diseases because characters like age, education, poverty and smoking rates can also impact health. Those factors were considered too.

Air quality trends were tracked from 1999 to 2015.