In Episode 40 of the forthcoming 2nd edition of Time Tunnel (Volume 2 of Time Crystal), the background radiation level inside a time bubble is measured as 2.73 microSievert per hour. This requires a little explanation.

Sievert is a measure of the risk to health from radiation. It is normally quoted in microSievert (usually abbreviated to microSv) per hour or milliSievert (mSv) per year. The higher the value, the greater the risk.

European Union (EU) regulations defining allowed exposure are given in Directive 96/29/Euratom – ionizing radiation (https://osha.europa.eu/en/legislation/directives/73), which lays down basic safety standards for the protection of the health of workers and the general public against the dangers arising from ionizing radiation. The regulations say (in part):

With regards to the protection of workers, the Directive defines dose limits for workers exposed to ionising radiation. The effective dose is limited to 100 mSv over a period of five consecutive years and must not exceed 50 mSv in one year. Young workers (under 18 years) may not be exposed to ionising radiation at work.

Thus the safe radiation level is regarded as 20 mSv per year. Assuming a worker works 2000 hours per year, this is equivalent to 10 microSv per hour.

According to a private communication to the author from the CERN Press Office:

The ambient dose rate on and near CERN sites is, in average, of about 0.085 microSv/h. The value is typical of such altitude and latitude.

CERN typically delivers about 0.01 mSv of effective dose of radiation to someone residing in the local area per year. This is less than 1% of the total annual dose of 3.7 mSv that individuals already receive on average, both naturally, through radioactive elements in soil and rocks or cosmic rays, and artificially through medical procedures.