The mercury is rising, the sun is shining and the blue sky is, well, it's turning kind of a grayish shade of brown.
It's Southern California's summertime paradox: The season is inviting, but the air can be downright unfriendly.
So, should you lace up your running shoes, or settle for the hammock?
Or, as joggers, speed walkers and bikers wonder as they see whole mountain ranges disappear behind gray curtains of smog: Are the positive benefits of exercise outweighed by the negative effects of pollution?
The answer, unfortunately, is no clearer than the sky.
The Air Quality Management District and some health experts suggest that you avoid just about every kind of vigorous exercise when it's very hot and smog rises to unhealthful levels--as it did on Thursday, when a first-stage smog alert was called for the San Gabriel and San Fernando valleys.
Heading the AQMD's list of activities to shun is jogging. Tennis, basketball, soccer and even water polo can also be harmful, said spokesman Bill Kelly.
That's because pollution can be bothersome, irritating and even painful, medical authorities warn.
But some experts are less strident in their warnings, suggesting that vigorous exercise is still acceptable even on the worst days if it is done early or late in the day when pollution levels tend to be lower. Or they suggest driving to the coast, where the air is cleaner.
Pollution can be dangerous for exercisers simply because they breathe much more of it.
"Someone running is getting 10 times as much air per minute as someone not exercising," said Dr. Steven Simons, a pulmonary specialist at Orthopaedic Hospital and a longtime marathon runner. Simons avoids the problem by running in the early morning.
Researchers worry that there could be long-term consequences of such intense contact with air pollutants, though major studies are incomplete.
"Sure we think about it," speed walker Lisa Houle said while doing laps around the Rose Bowl one recent afternoon as the smog index crept into the unhealthful range.
"We even talked about it before coming out," said Tori Boegh. "We knew it was smoggy because our eyes were burning."
Summer produces the worst air of the year because the bright sun and high temperatures combine with the toxic mix of pollutants to produce an unstable and troublesome gas called ozone.
"Ozone is like a poison," said Kelly. "It's not good for you. It's like any foreign substance. Your body will have a reaction to it. Your lungs and airways will get swollen and constricted. It will be harder to breathe."
Other symptoms sometimes suffered by exercisers include a pain or tightening in the chest, headaches and nausea.
Some feel the effects of impaired athletic function. "You just won't run as far or as fast," Kelly said. "You know. Some days you feel the rhythm and hop, and some days you don't. On smoggy days you will find you don't have that rhythm, you don't have that hop."
Dr. Henry Gong, chief of environmental health services at Rancho Los Amigos Hospital, said these effects are the result of potent chemical agents and the body's defensive response to them.
Upon entering the human airways, ozone begins reacting with the first thing it comes into contact with, usually the nerves and mucous membranes of the throat and lungs.
"Nerves are irritated," Gong said. "More mucus is produced as a defense against the ozone. It injures cells and cells respond by releasing [chemical] mediators. That begins the inflammation process."
"It does bother me," said speed skater Yello Sepulveda. Gesturing to the gray sky over the San Gabriel Mountains above Pasadena, Sepulveda said: "It's too much, I can feel it in my chest."
Another troublesome pollutant is carbon monoxide. It works its way into the lungs without producing any irritation. But from there, it enters the bloodstream. And then it is reluctant to leave.
The chemical binds tightly with hemoglobin in the blood, which would normally carry oxygen. The more carbon monoxide carried by the blood, the less oxygen is available to feed the body, Gong explained. That means reduced performance and endurance.
"Every day I do seven laps around [the Rose Bowl]," Sepulveda said. "Today, I'll do just four."
Long-term effects are more difficult to gauge. Some preliminary research indicates that there may be permanent loss of lung capacity from prolonged exposure to high levels of common air pollutants, several authorities said.
"It's hard to believe you can have chronic, day-to-day irritation and no permanent effect," said Dr. Allan V. Abbott, a professor of medicine at USC and a fitness expert.
Still, medical authorities are not prepared to say it is better to just stay inside and remain sedentary all day.
"Lack of exercise is one of our biggest health problems," Abbott said.
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Minimizing Your Risk * Exercise early in the morning or late at night when pollution levels are lowest.
* Avoid the smoggiest areas, such as the San Gabriel and San Fernando valleys, or exercise indoors.
* Plan your longest or hardest workouts for the weekend, when pollution levels are generally lower.
* Be aware of the pollution level, which is published daily in The Times. A PSI--pollution standard index--measure of 100 is the federal standard for unhealthful air. At a level of 138, the AQMD advises that exercise be avoided.