Fairfax County will tighten its rules governing the quarantine of pets exposed to rabies, county health officials announced yesterday.

The 30-day quarantine permitted in Fairfax for vaccinated pets that have come in contact with rabid animals will be increased on Monday to 90 days, the minimum recommended by federal and state officials, said Dr. Garth Dettinger, deputy county health director.

As of last night, 99 rabid raccoons and one skunk have been found in the county in the last 10 weeks, exposing more than 50 dogs and cats and more than a dozen humans to rabies. The current rabies outbreak in the Washington area is the worst in decades.

A Fairfax Station woman was undergoing rabies shots last night at Fairfax Hospital after an apparently rabid raccoon attacked her dog yesterday afternoon and she petted the dog with a cut hand, said Dettinger.

A skunk that attacked four dogs and sprayed two people on Tuesday near Braddock and Burke Lake Roads also was confirmed as rabid yesterday, he said.

The new quarantine rules were announced after Dettinger called state health officials in Richmond late yesterday in response to criticism from veterinarians who had said that the 30-day quarantine period was too short.

Dettinger said that the county had been following the state code in permitting a 30-day quarantine. State health officials confirmed yesterday that the code calls for a minimum 30 days, but said it had been superseded in 1980 by state health regulations requiring least a 90-day quarantine. Exposed pets that are unvaccinated, or those with expired inoculations, are required to spend six months in quarantine.

"There is no question about it, a 30-day quarantine is not good enough during an epidemic . . . ," said Dr. Robert C. Brown, president of the Northern Virginia Veterinary Medical Association, "especially when no vaccine is more than 85 to 90 percent effective."

At present, 42 dogs and three cats are undergoing 30-day to six-month home quarantines in the county. They must be confined at home or in a kennel, but are permitted out for walks on a leash on the owner's property. State health officials and Brown also criticized this procedure as being insufficiently confining.