A cloud of smoke from the Zaca fire that continues to cover Ventura County poses no immediate danger to residents, but air quality likely will worsen over the weekend and early next week, air pollution officials said Thursday.

A high-pressure system arriving over the next few days is expected to lower the smoke and fine particulates that can get into people's lungs and cause problems, said Kent Field, a meteorologist with the Ventura County Air Pollution Control District.

"As long as the smoke stays elevated, the air quality will remain in the moderate range," or about the same as a normal summer day, he said. "But it could get worse, especially Monday and Tuesday."

The cloud of smoke was 2,000 to 4,000 feet above ground Thursday, Field said.

People in the mountains and at other higher altitudes should limit outdoor activities, and those with asthma or other lung problems should avoid outdoor exercise if the air bothers them.

The 127,200-acre Zaca fire in Santa Barbara County continues to consume large swaths of wilderness on its eastward march and is expected to reach "back burns" that have been established inside Ventura County within days, said Maeton Freel, an information officer with the National Forest Service.

"The fire line is set slightly inside the county, so it will eventually cross the county line," Freel said.

On Thursday, the fire was 63 percent contained. It is burning north and east and charred more than 10,500 acres in a 12-hour period that ended Thursday morning.

Fighting the fire has cost $73.4 million so far. About 2,900 firefighting personnel remain part of the massive operation, trying to squelch a stubborn blaze that began July 4 when a ranch worker north of Los Olivos was grinding an irrigation pipe and a stray spark ignited some dry grass.

"The fire is still 22 miles west of Ojai," Freel said, "and 11 miles west of Matilija Canyon Road, where there are some homes."

Portions of Highway 33 north of Ojai might be closed over the next few days so fire crews can use the highway as a staging area, Freel said.

Fire crews, he said, are concentrating on keeping the fire west of Highway 33 and from advancing toward New Cuyama at the northern end and toward Montecito and Carpinteria to the south.