MARIE found that radiation levels in orbit above Mars are 2.5 times higher than at the International Space Station. The average daily dose was about 220 μGy (22 mrad) – equivalent to 0.08 Gy per year.[46] A three-year exposure to such levels would be close to the safety limits currently adopted by NASA.[citation needed] Levels at the Martian surface would be somewhat lower and might vary significantly at different locations depending on altitude and local magnetic fields. Building living quarters underground (possibly in Martian lava tubes which are already present) would significantly lower the colonists' exposure to radiation.