If you see some low flying helicopters circling in the sky, don’t freak out! The National Nuclear Security Administration announced a low-flying helicopter will be measuring “naturally occurring background radiation” over an area covering 13 square miles from Tuesday through Friday.

The flights will only happen during daylight hours and will be low and about 80 mph. According to a statement from NNSA, “The measurement of naturally occurring radiation to establish baseline levels is a normal part of security and emergency preparedness.”

Although the marathon is not mentioned in the statement, the government has been measuring radiation over Boston for the past several years in preparation for the race.

Image from NNSA