In response to reports that poison ivy has become stronger and more prevalent lately, a U.S. dermatologist sheds light on the plant with the aim of encouraging a safer outdoor season.

The number of cases or severity has not changed much over the past 15 years, says Dr. David Adams of Pennsylvania State University at Hershey.

One in four individuals will not have any reaction upon exposure, according to Dr. Adams.

What causes the rash is a substance called urushiol oil, which exists within the plant's leaves, stem and roots, he says.

Importantly, the rash doesn't develop as a result of brushing your skin against the leaves.

"You have to actually break the leaves, stem or root to get the urushiol oil on you," says Dr. Adams.

What's more the irritation takes time to develop, depending on prior exposure to urushiol oil.

Upon first exposure, symptoms often take seven to 10 days to appear, however, those who have already been exposed normally see a rash within a day or two after contact.

Normally, the rash is not contagious nor does it spread by scratching, according to Dr. Adams.

Burning yard waste and debris releases the oil into the air, so stay away from yard fires that may contain poison ivy plants, for exposure to the smoke could cause any exposed skin to swell and itch.

"The most common method, though, is that someone is pulling out weeds and then they rub an eyelid or something," says Dr. Adams.

He recommends washing clothing and garden tools after use, as the oil remains on objects and retains its potency for long periods of time.

While the most severe cases he sees occur in summertime, Dr. Adams -- himself a lifelong outdoorsman -- says there is a small spike in cases during the winter holidays.

"What happens is that people buy a live Christmas tree, and there are dead vines on it that they rip off, not knowing what they are," says Dr. Adams. "The urushiol oil inside is still viable."

Mild, localized rashes are usually treatable by means of topical over-the-counter medications such as cortisone cream and calamine lotion, available in most local pharmacies.

See your doctor if the itching becomes unbearable, says Dr. Adams, emphasizing that certain cases require prescription creams and sometimes orally administered steroids such as prednisone.