So far this year, fires have destroyed hundreds of homes and tens of thousands of acres in California. Thanks to the state's four-years-and-counting drought, those flames are spreading fast and far. But what travels even faster are the billows of smoke from those fires—which means a fire's impact extends far beyond its perimeter.

When smoke blows unbidden, it threatens the lungs of Californians many miles from a fire. Its tiny particles cause all manner of respiratory problems, and can even mess with your heart. Currently, moist weather is keeping a lot of the bad air from three huge California fires in check, but things are set to warm up. That could have many more Californians soon feeling the burn in their lungs.

What you see as smoke is actually microscopic organic matter, burned down and churned up into the air. Smoke makes you cough because the tiniest pieces of organic matter—called 2.5 micrometer particulates—are getting into your lungs. At about 1/35 the diameter of a human hair, 2.5µm particulates are so small that they slip through past all the hair and mucus your body uses as a defense.

Once in your lungs, those particulates can cause everything from coughing to asthma to chronic lung disease. That's not all. The 2.5µm particulates also can slip into and contaminate your bloodstream. Breathing smoke can even give you a heart attack. "If you can smell smoke, then basically you’re breathing it," says Sylvia Vanderspek, chief air quality planner at the California Air Resources Board. Kinda puts a damper on all those great campfire memories.

Smoke blows with the wind, and right now in California, the wind is delivering wildfire smoke from more than 10 fires—including those three biggies—to millions of Californians. Typically, airflow enters California from the Pacific Ocean into the San Francisco Bay, then into the Great Central Valley. There, the flow splits, going north to Redding, and south toward Fresno and Bakersfield. Several large fires currently are burning in this wind path—including one of the state's most destructive fires, as well as one of the state's largest ever.

According to the feds, an average human can breathe in about 35 micrograms of 2.5µm particulates over 24 hours before health problems set in. The state's air quality board monitors those levels statewide, and some of the highest—currently between 10 and 34 micrograms per day—are registering hundreds of miles away from the huge Valley, Butte, and Rough fires.

That's not as bad as it could be, because currently, the state is sitting under a low pressure system. "Which is a boon," says Dar Mims, meteorologist with the California Air Resources Board. "Because any moisture helps in knocking down some of the particulates."

But none of the three big fires are even halfway contained, and this week's forecast calls for clouds to clear and temperatures to rise. Worst of all, that high pressure system probably will bring atmospheric inversions. "Things become more problematic," says Mims, "because atmospheric inversions will strengthen and trap particulates." With particulates being pushed down by heavy, upper atmospheric air, smoke settles into low lying areas, like the 14.4 million acres of the Central Valley that four million people call home.

Beyond the fire season, California might get some much needed rain from this year's strong El Niño. Then again, it might not. Either way, Californians probably won't be breathing easy for a long time.