Within a single species, individual differences make some individuals more resistant and some more susceptible to chemical toxicity. The gender of the animal can affect sensitivity as well. Women are more susceptible to alcohol than men. Age can also affect toxicity. Babies and small children have less well developed immune systems and higher respiration rates than adults, making them generally more susceptible.
Nutrition can also affect susceptibility.
How the body metabolizes a chemical also affects toxicity. The liver typically metabolizes substances into less toxic forms, but it can sometimes make a chemical more toxic. This is true of some environmental carcinogens, which are not carcinogenic in and of themselves, but become so when processed by the liver. Many other factors also affect toxicity. By the same token, barium is a toxic heavy metal, yet barium sulfate, which is used in x-ray diagnosis, is fairly innocuous because it is so insoluble that it is not absorbed across the wall of the gastrointestinal tract.