Inhaling an irritant of some kind, over a lengthy period of years, increases the chance of precancerous cell mutation, which increases the chance of cancer. Some precancerous lesions form in the absence of any carcinogen. This would be either random chance, or an unknown or less obvious carcinogen. An older population as a whole increases the apparent frequency of cancers. Live long enough and individual odds of having some form of cancer are more than even.
I hate to sound even remotely like some of these leftist environmentalists, but something has changed. Cancers in pet dogs have gone through the roof in the past decade or so. Excessive vaccinations and/or chemical exposure (lawn chemicals and internal or external flea and tick treatments, primarily) are being blamed there.
Many cancer treatments in humans originate with canine veterinary medicine. The two, human and canine, respond similarly to treatment and are prone to many of the same cancers. So, it’s reasonable to theorize that the cause(s) might be similar as well.
i have known many good young dogs who have died in the past year. I am rethinking heartowrm and flea treatments