Second of three parts

LITTLE GASPARILLA ISLAND, Fla. — All Susan Leydon has to do is stick her head outside and take a deep breath of sea air, and she can tell if her 10-year-old son is about to get sick.

If she coughs or feels a tickle in the back of her throat, she lays down the law: No playing on the beach. No, not even in the yard. Come back inside. Now.

The Leydons thought they found paradise a decade ago when they moved from Massachusetts to this narrow barrier island, reachable only by boat, with gentle surf and balmy air.

Now, they fear the sea has turned on them. The dread takes hold whenever purplish-red algae stain the crystal waters of Florida's Gulf Coast. The blooms send waves of stinking dead fish ashore and assault every nostril on the island with something worse. The algae produce an arsenal of toxins carried ashore by the sea breeze.

"I have to pull my shirt up and over my mouth or I'll be coughing and hacking," said Leydon, 42, an energetic mother of three who walks the beach every morning.

Her husband, Richard, a 46-year-old building contractor, said the wind off the gulf makes him feel like he's spent too much time in an over-chlorinated pool. His chest tightens and he grows short of breath. His throat feels scratchy, his eyes burn, his head throbs.

Yet their symptoms are mild compared with those of their son, also named Richard. He suffers from asthma and recurring sinus infections. When the toxic breeze blows, he keeps himself — and his parents — up all night, coughing until he vomits.

On weekends, the Leydons escape inland. They drive three hours to Orlando so their son can play outside without getting sick. "We have to get away from it," Richard Leydon said.

Harmful algae blooms have occurred for ages. Yet, what was once a freak of nature has become commonplace. These outbreaks, often called "red tides," are occurring more often, showing up in new places, lasting longer and intensifying.

Scientists believe that partially treated human sewage and farm runoff are generally responsible for the worldwide spread of algae blooms.

In essence, they think human activity is force-feeding the oceans with the basic ingredients of fertilizer — nitrogen, phosphorous and iron — that make these microscopic aquatic plants flourish. But when they focus on individual blooms, scientists often have not been able to pinpoint the causes.

People who have spent many years on Little Gasparilla Island and in other Florida Gulf Coast communities said red tides used to show up once in a decade. Now, they occur almost every year.

The last red tide, which ended in mid-February, peppered Florida's western coast with its fiery breath for 13 months, stubbornly refusing to dissipate despite three hurricanes.

The culprit is a microorganism known as Karenia brevis. Each Karenia cell is a microscopic poison factory, pumping out toxins collectively known as brevetoxin. They are absorbed into the food chain by scallops, oysters or other popular seafood.

Brevetoxins also get into the air. They collect on the surface of bubbles and concentrate in sea foam and on dead fish.

When the bubbles burst, brevetoxins are flung into the air and carried by the wind. If inhaled, most particles lodge in the nose and throat, but some are drawn deep into the lungs. People don't have to set foot in the ocean or even on the beach to experience a red tide. It comes to them.

Hundreds of visitors from the Midwest and New England have posted questions and complaints on Web sites, seeking to learn why after a short vacation on Sarasota's beach they suffered weeks of coughing, bronchial infections, dizziness, lethargy and other symptoms.

Brevetoxins don't directly cause severe respiratory ailments. Instead, researchers believe, they make people more vulnerable to respiratory illness by inflaming their sinuses and suppressing their immune systems. Studies show that besides reducing the ability of mucus to clear airways, the toxins may hamper infection-fighting white blood cells.

Dr. Lora Fleming, a University of Miami epidemiologist and physician, isn't convinced that people on the beach can inhale enough to suffer serious neurological symptoms but thinks there may be something to the complaints she hears from surfers.

John Purdy, a former Manatee County lifeguard, was paddling his surfboard over a wave last fall when some sea foam lifted off the water and into his mouth just as he was gulping for air.

"I felt like I inhaled a garbage bag," said Purdy, 33, a former high-school swimming champion. "It locked up my lungs and throat like a paralysis."

He couldn't take a breath. The seconds ticked by.

"I was thinking, 'Is this the way it's going to end?' "

Eventually, he managed to inhale a little air. He made his way to shore but didn't feel much better until emergency medical technicians hooked him up to oxygen.

"It was the closest thing I've had to a near-death experience," he said.

A television suspended from the ceiling at Mote Marine Laboratory in Sarasota plays a public-service announcement sponsored by state and federal agencies, offering hints for dealing with red tide.

"If you are going to the beach for a short trip, go to your local drugstore and buy a face mask, like the ones painters wear," the narrator says. "But remember, these masks are only effective for a short time."

Red tides have become a staple of the daily reports on surf conditions posted on the lifeguard tower. The sign reads: "Some Red Tide Coughs. Sneezes. Dead Fishes."

A few extra words were scribbled in chalk in the margin: "Can't do anything about (it)."

Not that people haven't tried.

In one experiment, researchers from Mote laboratory sprayed a slurry of clay onto the murky water in an effort to smother and sink the offending organisms. Another experiment in the works would sterilize algae patches with injections of ozone. Such remedies can be problematic. Not only do they kill the harmful algae, but they also wipe out every living thing in the vicinity.

Cynthia Heil, a senior state scientist, said there is no evidence that pollution from agriculture or development spawns red tides, although coastal pollution may intensify or prolong the outbreaks.

Heil and a team of university scientists in Florida have published a study theorizing that iron-rich dust from Africa's Sahara Desert drifts across the Atlantic and triggers a natural process that stimulates the harmful algae bloom off Florida's Gulf Coast.

"The timing sure matches up with blooms," Heil said. "We know it has to contribute to enriching seawater with iron and nitrogen."

Susan and Richard Leydon were keeping their dog, a sheltie, inside, along with their son, Richard. The air conditioner was going full blast.

The boy has spent nearly his entire life on the island and was among the first in the family to develop symptoms. His most common ailment is a dry cough, which he says makes him sound like a barking seal.

The airborne irritants have also triggered recurring sinus infections and asthma. On a few occasions, during intense and prolonged red tides, Richard has been diagnosed with bronchitis and even pneumonia, which kept him out of school for more than a month.

The Leydons said they have consulted with several specialists over the years and spent thousands of dollars on tests trying to figure out what is making their son sick.

They worry about the price their son is paying for their decision to move to Florida.

"Is Richard going to have lung scarring and long-term problems?" his father asked. "I need to know."

The conversation in the Leydon household focused on two topics, as it often does during red-tide outbreaks. One was where to flee for the weekend. The other was whether they should move, for good.

"Do we have to sell our house because paradise is killing us?" Susan Leydon asked.

Times staff writer Usha Lee McFarling contributed to this report.