(CNN) -- The scenes from the California wildfires are horrifying enough, with windblown walls of flames destroying homes and buildings in their path, but health experts say the smoke can be even more dangerous.
Particles in smoke can cause serious health problems, especially in children and older adults.
Plumes of smoke can be seen from NASA satellites and are so thick that they look like rain on Doppler weather radar.
But the real concerns are the particles and gases in the smoke. When a fire burns it generates carbon dioxide, water vapor, carbon monoxide, particulate matter and other chemicals.
And the wind can carry those particles to areas far from the fires. See the NASA photograph of smoke from the fires
When you breathe the smoke, the tiny particles burrow deep into the lungs, causing serious irritation, mucous build-up and breathing problems.
Most masks don't really help, because the particles are so tiny they pass through the filters, according to the Centers for Disease Control and Prevention. Watch why smoke is a threat even to healthy people »
I-Reporter Michele Hemming said "the air is almost unbreathable" in San Marcos.
Heather Storke told CNN that her grandmother had to be evacuated from a nursing home in Poway, which is north of San Diego.
She said her parents got a call from the nursing home at about 6 a.m. (9 a.m. ET).
"My parents quickly went to go get my Gram. The closer the got to Poway, the thick cloud of smoke made it like pitch-black night. When they arrived, there was a fleet of school buses waiting to evacuate the Center's residents whose families were unable to get there," she said.
"The air was horrible, difficult to breathe and burning eyes, so they put a blanket over my Gram's face on the way to the car," Storke said.
She said the nursing home staff had packed her grandmother's things, along with her dialysis schedule, a five-day supply of her medicines and new prescriptions in case the evacuation lasted longer than that.
Storke said it took her parents four hours to drive from Oceanside to Poway and back, a trip that normally takes 1½ hours.
The CDC said that people with heart and lung disease, children and older adults are most vulnerable, but even healthy adults can suffer symptoms when smoke levels are high enough.
It said smoke can cause:
• A scratchy throat
• Irritated sinuses
• Shortness of breath
• Chest pain
• Stinging eyes
• A runny nose
• Asthma exacerbations
The government's Air Quality Index shows "unhealthy" pollution levels in much of Southern California. That means that people may experience these symptoms.
CNN Chief Medical Correspondent Dr. Sanjay Gupta said those problems can last for weeks after the fire.
He said the best thing to do if you smell smoke is to stay inside.
The CDC recommends keeping your doors and windows shut. If you have an air conditioner, run it with the fresh air intake closed so you don't pull in smoke from the outside. If you don't have an air conditioner and it's too hot to keep the windows shut, you should evacuate to a place with cleaner air.
Smoking and using candles, gas stoves or fireplaces can add to indoor air pollution and vacuuming stirs up particles that are already in the house, so the CDC said to avoid doing those things. E-mail to a friend