Hi Arun,

You can find the limits in the following document (published in 2003). I dont know however, if this has been updated.

http://www.who.int/peh-emf/meetings/archive/en/keynote3ng.pdf

Cheers

Thank You Ahmad

this summarizes the EPA guidelines - even very low levels can cause biochemical changes

http://www.epa.gov/radiation/understand/health_effects.html

and here are the NRC dose limits for radiation workers

http://www.nrc.gov/about-nrc/radiation/health-effects/info.html#dose

some think very low doses might be beneficial

http://newscenter.lbl.gov/news-releases/2011/12/20/low-dose-radiation/

For the purpose of protection against ionizing radiation exposures, the International Commission on Radiological Protection (ICRP) has classified all health effects into stochastic effects and tissue reactions (formerly called deterministic or nonstochastic effects). On one hand, the effective dose limit aims to keep the risk of stochastic effects below an acceptable level, where a clear distinction between "safe" and "dangerous" cannot be derived due to he probabilistic nature of stochastic effects and the properties of the linear nonthreshold model. On the other hand, the equivalent dose limits aims to prevent the occurrence of the tissue reactions, where no effects are assumed to occur below a threshold. Currently, threshold dose is defined as dose that causes the effects of interest in 1% of exposed individuals. The dose thresholds depends on the types of effects, severity, and dose rate, although the same threshold is considered for cataracts and circulatory disease independent of dose rate.