A hospital-based case-control study of companion dogs examined the risk of developing canine malignant lymphoma associated with the use of chemicals in and about the home. The study indicated that owners in households with dogs that developed malignant lymphoma applied 2,4-dichlorophenoxyacetic acid (2,4-D) herbicides to their lawn and/or employed commercial lawn care companies to treat their yard significantly more frequently than control owners(odds ratio = 1.3).

In addition, the risk of canine malignant lymphoma rose to a twofold excess with four or more yearly owner applications of 2,4-D. The findings in this study are consistent with occupational studies in humans, which have reported modest associations between agricultural exposure to 2,4-D and increased risk of non-Hodgkin's lymphoma, the history and epidemiology of which are similar to those of canine malignant lymphoma.

Insecticides Associated with Bladder Cancer in Dogs - A statistically significant association between exposure to topical flea and tick dips, and the occurrence of bladder cancer in dogs has been found. The risk of bladder cancer was increased further in dogs living in proximity to areas sprayed regularly with insecticides for mosquitoes. An increased rate of bladder cancer in humans has recently been reported (Journal of Toxicology and Environmental Health, 1989(28): 407-414). 

What about flea sprays and shampoo?

Excerpt from Catherine O'Driscoll's writing

"Did you know that many flea collars contain carbaryl? Do you know what carbaryl has been shown to do to animals in laboratories? It causes cancer, mutates cells, and causes birth defects. How safe is it for your dog to wear a flea collar around his neck for up to four months? And why do the manufacturers tell us to stop our children from touching the collar? And the flea sprays - read the instructions. Are you told not to breathe in whilst spraying your dog? Can you tell your dog not to breathe? And what's in all the flea killing shampoos? One of them, at least, contains a chemical that has been shown to cause cancer if taken in through the skin!"