Subsequent studies of different doses during various gestation times confirmed these findings, they said. The authors concluded that simultaneous occupational exposure to excessive levels of toluene and noise increased the probability of developing hearing loss. The level of hearing loss was much greater in workers exposed to both hazards than would be predicted by adding the effect of each agent. Humans almost never encounter substances one at a time. They not only cause cancer but they affect the nervous, endocrine and immune systems in ways that are poorly understood.
Risk assessment has no way to take into account such complex and cumulative interactions.
The only approach that can consider all these effects together is prevention, the principle of precautionary action.