When Heather Goddard's 8-year-old rescue dog, Clover, collapsed during a walk, Goddard rushed her pet to the vet, where X-rays revealed that Clover had dark spots on her lungs.

The veterinarian diagnosed the dog with lung cancer, which was likely caused by inhaling and ingesting toxic chemicals from secondhand smoke.

Doctors told Goddard that the cancer was untreatable, and Clover was euthanized.

After Clover’s death — and after being diagnosed with chronic obstructive pulmonary disease herself — Goddard quit smoking, and now she hopes sharing her story will encourage other pet owners to protect their animals from passive smoke.

"It brings a tear to my eye just thinking about the damage my smoking was doing to those around me," she told the Daily Mail. "Before I quit, I never thought twice about smoking around my grandchildren or my pets."

The dangers of secondhand smoke

Dogs and cats have lungs very similar to our own, and passive smoke can be just as damaging to their lungs as it is to ours.

But pets don't just inhale toxic chemicals from cigarette smoke — they also ingest them.

Cats and dogs lie on carpets and furniture that have been exposed to carcinogenic substances, and the particles can get trapped in their fur, only to be ingested when the animal grooms itself.

Experts say one reason cats are so vulnerable to tobacco smoke is because they spend 50 percent of their waking hours grooming, which can expose delicate oral tissues to hazardous amounts of carcinogens.

"In most households, your cats and dogs can’t get away from polluted air unless they are fortunate enough to have a doggie door that leads outdoors," writes veterinarian Dr. Karen Becker. "Most animals are trapped, victims of their owners’ habits, and opening a window is not enough."

Cats that live with smokers have been found to have nicotine and other toxins in their urine, according to research by the University of Minnesota.

Numerous studies have linked secondhand smoke to cancer in cats, and a 2002 Tufts University study found that cats living with smokers are twice as likely to develop malignant lymphoma, a disease that kills three out of four cats within a year of diagnosis.

Dogs are also at risk from passive smoke. A study published in the American Journal of Epidemiology found that dogs in smoking households have a 60 percent greater risk of developing lung cancer.

The length of a dog's muzzle plays a role in the type of cancer the canine is likely to develop from secondhand smoke.

Those with short or medium-length muzzles, such as boxers and bulldogs, have shorter nasal passages, making lung cancer more common. Canines with longer noses, such as German shepherds and retrievers, are more likely to develop nasal and sinus cancers.

The good news is that nearly one-third of smokers with pets said information about the dangers of secondhand smoke to their pets would motivate them to kick the habit.

"If my story stops one more dog, cat or other pet dying from the effects of secondhand smoke then it’s been worth telling," Goddard said.

Related on MNN: