A major challenge for the toxicologist involved in safety evaluation o
f chemical mixtures is to test the hypothesis that as a rule exposure
to mixtures of chemicals at (low) non-toxic doses of the individual ch
emicals is of no health concern. A series of repeated dose studies in
rats with defined mixtures of chemicals with the same or different tar
get organs revealed that exposure to a combination of chemicals compar
ed with exposure to the individual compounds did not constitute an evi
dently increased hazard, provided each chemical was administered at a
level similar to, or slightly lower than, its own 'No-Observed-Adverse
-Effect-Level'. The results of subacute oral toxicity studies in rats
with defined mixtures of nephrotoxicants with similar mode of action u
nderlined the applicability of the additivity assumption for a mixture
of chemicals with simple similar action. Safety evaluation of complex
chemical mixtures is a challenge that can be tackled as follows: firs
t, identify the (e.g, ten) most risky chemicals in the mixture, and, s
econd, assess the hazard and the potential health risk of the mixture
of the most risky chemicals, using procedures developed for defined mi
xtures. To identify interactions between individual compounds, a most
promising testing strategy appeared to be a statistical approach using
a fractional two-level factorial design. A challenge for today and th
e future is to gradually substitute mixture-oriented (real life-orient
ed) standard setting for (unrealistic) single chemical-oriented standa
rd setting.