San Francisco ? If you’re allergic to cat or dog hair, good luck getting away from it in your home — whether or not you have pets.

Highly mobile cat and dog allergens are universally present in all homes, and the living room sofa has the highest concentration, according to a new study published in the Journal of Allergy & Clinical Immunology.

Although a dog or cat had lived in slightly less than half — 49.1 percent — of homes in the last six months, dog allergen was detected in 100 percent of the homes and cat allergen was found in 99.9 percent.

“The surprise was the magnitude of the exposure, and significant levels were present in homes even without dogs and cats,” said Dr. Darryl Zeldin, co-author of the study and senior investigator at the National Institute of Environmental Health Sciences.

The allergen levels were high enough to increase the risk of allergies and asthma, making avoidance difficult for people predisposed to the diseases, Zeldin said.

“Traditionally, the approach to allergy and asthma is to avoid the environmental triggers,” Zeldin said. “For cat and dog allergens, you can tell people not to have a dog or cat in the house and that lowers it significantly, but even if they don’t have a dog or cat in the house, levels are still going to be high enough to cause symptoms.”

The sofa is a hot spot for indoor allergens, not just because it’s where pets spend a lot of time, but because it comes in contact with clothes worn outside the home, which can transport allergens inside, the study said. Even so, dog and cat allergens were fairly equally distributed across all parts of the house, Zeldin said.

Allergen levels also correlated to socioeconomic and geographic factors, with the West showing the highest levels for both dogs and cats. Owner-occupied homes tended to have higher levels than renter-occupied homes, and affluent households had more dog allergen than those in poorer neighborhoods, Zeldin said.

“It’s the community that’s really a major source of allergen in homes without dogs and cats,” he said. “That’s difficult because how do you intervene? You can’t tell (other) people not to have dogs and cats because you have asthma.”

Among people sensitive to animal allergies, cats appear to be the biggest offenders, said Dr. Jonathan Field, director of the pediatric allergy, asthma and immunology clinic at New York University Medical Center/Bellevue.

Even so, the severity varies person to person and can depend on how much environmental control is done to contain the culprit, he said.

“It’s ubiquitous,” Field said. “It’s all over the place, and it does tend to stick around. There’s no place that’s totally free of cat antigen.”

Those who are bothered even after removing the source are wise to use an air filter, especially a HEPA filter, Field said. And the most severe cases may consider immunotherapy, also known as allergy shots, to control the symptoms, he said.

Ultimately, the study showed that dog and cat allergens are virtually impossible to avoid, Zeldin said, noting they’re especially prevalent in schools and among school-age children who may transport them on their clothes.

“Your best bet for environmental control is to not have a dog or cat in the house and live in a community where dog and cat ownership is very uncommon,” he said.