A convenient method of specifying what is meant by low-level electric and magnetic fields is to examine human exposure standards. Exposure standards have not been static, but have changed with time. Safety levels have usually been lowered as knowledge of the effects of electric and magnetic fields has increased. Furthermore, exposure standards are not uniform throughout the world; safety levels are generally significantly lower in eastern European countries compared with western Europe and the USA. The following listing gives accepted or proposed exposure standards for various types of radiation in several countries.115, 116