Not much frightens Brad Precosky. After all, the Anchorage mountain runner has won Seward's Mount Marathon race six times, negotiating perilous downhills with a graceful controlled glide.

But a tall green plant often edging onto some of Precosky's favorite mountain running trails scares the bejesus out of him.

Hairy stalks of cow parsnip or pushki (poosh-kee) can top 8 feet this time of year, with a stately umbrella of white flowers when in bloom.

"I don't know how it works, but I started reacting to it three years ago. Now I'm terrified of it.

"I wear pants everywhere. The bad part lasts for at least a week -- several weeks until the itching phase fades. A cold ice cube is the only thing I've found that makes the scratching go away for a little while."

The source of the rash that can cause scarring is the chemical furanocoumarin, found in the sap and outer hairs of cow parsnip. Plants use the photo-toxic furanocoumarin to protect themselves from fungus attacks.

Dermatologists say the other necessary ingredient is the ultraviolet radiation of sunlight, though Precosky thinks heat may be the larger factor.

"If you're exposed and go into sauna, it seems to explode," he said

The itch can be awful. Sometimes the infected even wear gloves to dull the effects of the scratching.

"Think of it like poison ivy or poison oak," said Roy Howard, a physician's assistant at Independence Park Medical Services who has treated it for years. "The chemical makes it itch, and you will scratch it.

"Once a group of cells responds, it's easier for next group of cells to respond, so it's basically a cascade.

"And once people are sensitized to its effects, it's much easier for it to flare again."

To avoid a recurrence, hikers should cover up with long pants and long sleeves -- even gloves in rare cases. Even the dust from the dried sap of cow parsnip can infect some people, Howard said.

"That's the insidious part of it," Precosky said. "The first time you're exposed to it, you're susceptible -- and then you just get more and more susceptible."

Howard often recommends hikers sensitive to cow parsnip apply a cream like Stokoguard before heading afield to block the irritant from penetrating the skin.

The alternative can be nasty -- a rash of blisters several inches long and filled with fluid, redness or darkened pigmentation

Warm sunny weather has made this summer ideal for cow parsnip.

"We've got parsnip so big it's hanging over the trail," said Carl Skustad of Chugach National Forest. "It's bad."

Trails such as Johnson Pass, Resurrection Pass and Crow Pass have sections thick with cow parsnip.

"Crow Pass has historically been bad for it," said Harlow Robinson, the Crow Pass Crossing champion in 2001 and 2002. "I still have some scars from a couple of years ago.

"If you're the first one through, you're bound to get a dose of it."

Misery is likely to follow.

"I'm covered right now from out at Kincaid," said prominent Anchorage triathlete Shannon Donley. "I've had it from Crow Pass and on those really bushwacky things, but I wasn't thinking about it at Kincaid.

"But I did the Hammerman (Triathlon) at Kincaid, and there's some single track where I must have picked it up."

While backcountry runners, bikers and hikers may be the likeliest victims, they aren't the only vulnerable targets, Howard said. Lime rind contains similar furanocoumarins, he said, and bartenders working outdoors on sunny summer days can flare up too.

Reach reporter Mike Campbell at mcampbell@adn.com or 257-4329.

Cow Parsnip

• Characteristics: Grows to more than 8 feet tall, typically flowering in July with its seeds ripening in August. Prefers moist soil.

• Other Names: Pushki, Indian celery, hogweed, celery sorters disease.

• Distribution: Throughout the U.S. except for the Gulf Coast.

• Skin Reaction: Cow parsnip juices contain a phototoxin that acts on contact with skin, triggered by exposure to ultraviolet light. Reaction differs sharply among individuals — from next to nothing to a mild rash to blistering and severe dermatitis, depending on the sensitivity of the individual. Typically, heat intensifies the symptoms. The light-triggered reaction happens quickly.

• Defenses: Most obvious is to avoid contact by wearing long pants and shorts — even gloves. If contact is made, try washing the area with soap and water. Some dermatologists suggest a hydrocortisone cream to reduce swelling and itching, along with avoiding further exposure the sun.

• Duration: Rashes and the itching that accompanies them often last longer than a week. Discoloration can persist several weeks. Scarring can result from the worst cases.

• What’s Good About It?: Some scientists are looking at furanocoumarin’s potential for treating certain cancers as well as psoriasis and other skin disorders.