Many years ago I attended a Coloquim given by the head of the toxicology department at Carnegie Melon. He described in considerable detail, the infrastructure they used to protect their animals from environmental toxins. The life expectancy of their lab mice was 50% longer than at most most other labs. When asked about the incidence of cancer in their controls he replied that in his twenty years at CM he had not observed a single malignancy in a control subject.
OTOH, in some labs the mice breathe city air and drink tap water.
There may even be chrome or nickle plating in their cages.