In recent centuries these changes have reached a new dimension in terms of both
quality and quantity. Through human activity the environment has been confronted
with totally new substances that did not previously ex ist (x enobiotics, many radionuclides)
and potentially harmful substances released in quantities unthinkable in the past
(heavy metals, natural radionuclides). W hat is more, these new stressors usually have
a multiplying effect, i.e. they are added to the effects of natural systems, or they themselves
act in combination, with the result that the “ tolerance level” of the organisms’
ability to cope or to adjust to them is ex ceeded (Oehlmann and Markert, 1999)