TOXNET (Toxicology Data Network) is a collection of databases on toxicology, hazardous chemicals, environmental health, and toxic releases. You can search individual databases, or all of them together. From the US National Library of Medicine.
Some of the databases:
HSDB: Hazardous Substances Data Bank
TOXLINE: Toxicology Literature Online: References from the toxicology literature
LactMed: Drugs and Lactation: drugs to which lactating mothers might be exposed.
The World Health Organization (WHO) has created several documents on environmental variables that are freely available.