Bottom line on cancer. Some individuals are genetically predisposed to certain types of cancer, or are "sensitive" to certain types of chemicals that are known to be carcinogenic. Not only is cancer on the rise, but so is juvenile diabetes and any number of behavioral disorders that statistically may be linked to changes in the American food processing and preparation cycle, as well as increases in chemical usage in everyday life.
Better living through chemistry has disrupted our environment, our diet, and the metabolisms of many of the food animals and plants that we eat. Many of the implications are buried in endless scientific debate and pressure from a variety of industries. What a surprise. BTW, all of this is also linked to obesity and a variety of allergic and immune suppression disorders.
Photographers have been in the darkroom now for over a century and a half. Early on, things like mercury were a real hazard. For the past 50 years, the photographer that did not mix up specialized reducers and intensifiers has been at little risk of any chemical health issues. Yes, some people become sensitized to certain chemicals. This is not unusual, for even farmers become sensitized to fertilizers and other chemicals they use. Again, bottom line, the rates of cancer for the photographic industry are not historically increased over the general population. Todays stock chemicals are even more harmless than the custom stuff many mixed up a generation or so ago. Most photo chemicals break down to elemental salts, and are relatively harmless in the environment. Silver recovery is easy, see some threads here.
Quit worrying. There is some evidence that continued stress sets up certain physiological conditions that can lead to opportunistic diseases...