The San Fernando Valley saw bad air quality Monday morning as a small brush fire burned in the Hansen Dam area.

Air quality in other parts of Los Angeles County, including the San Gabriel Valley and the coastal communities of L.A. County, were also recorded at levels unsafe for sensitive groups due to higher levels of fine particles in the air, according to the South Coast Air Quality Management District’s map.

The particles, 30 times thinner than a strand of hair, can travel into the respiratory tract and reach the lungs.

Those with heart or lung disease, older adults and children were advised to reduce prolonged or heavy exertion and try to avoid the outdoors, according to health officials.

Exposure to the particles can cause eye, nose, throat and lung irritation, and symptoms can include coughing, sneezing, a runny nose and shortness of breath. In some cases, it can also affect lung function and worsen medical conditions like asthma and heart disease.

The particles usually come from vehicle exhausts, but can also come from forest and grass fires.

Southern California has had numerous wildfires in October and November. Together, the blazes ripped through thousands of acres, sending plumes of smoke billowing over communities.

The slow-moving brush fire in the Hansen Dam area erupted just before 1 a.m. Monday and burned for hours. Firefighters put out the fire by 11:30 a.m. after it burned about 3 acres.

Heavy fog and the fire’s remote location made it difficult for firefighting aircraft to drop water over the flames, and crews had to return at first light, according to the Los Angeles Fire Department.

Two days earlier, the Barham Fire ripped through 80 acres in Hollywood Hills, filling the sky over the Warner Bros. Studios and the Hollywood Sign with thick smoke.