Acute radiation sickness (hair loss, cell loss, thyroid malfunction, skin burn, etc.) is related to effective doses above 500 mSv. However, prolonged exposure to much lower doses are related to increased risk of cancer and other sickness. Exactly what level is safe is not known and there may not be a safe level. Radiation protection regulations are generally based on the principle that radiation’s effects should be considered to be proportional to the dose an individual receives, regardless of how small the dose is.
Canadians receive an average effective dose from our surroundings (cosmic, surrounding geology and food) of about 1.8 mSv per year (worldwide average 2.4 mSv). This will be higher in areas with uranium-mineralized rock in the local geology, where uranium mining is more likely to take place. To this background level, you can add the dose from any medical scans you may have (dental: 0,01 mSv, mammogram: 3 mSv, abdominal CT: 20 mSv).
Health Canada considers an effective dose of 1 mSv per year, above the background and medical exposures, safe for the general population.