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Comparing the culprits:
|History of contact||No previous exposure required. Everyone can be affected if sufficiently exposed.||Prior exposure and sensitization to poison ivy required.
Only 50-85 percent of the population will ever develop an immune response to poison ivy (and to chemically related poison oak and poison sumac).
It may take repeated exposures to develop sensitivity, though for many, one touch is enough.
|Onset||First exposure – redness within 24 hours, blisters for several days.|
Subsequent exposures – same reaction as first exposure.
|First sensitizing exposure readies the immune system to respond. |
Subsequent exposures – symptoms occur in several hours to 2 days.
Because poison ivy's reactive oil can last for months on clothing, pet fur and other surfaces, exposure can occur repeatedly.
|Location on body||Limited to areas exposed to sun.||Can occur anywhere poison ivy's reactive oil contacts the skin; often transferred by hands or clothing to areas "where the sun don't shine."|
|Symptoms||Burning pain, which is short-lived.||Itching, which can last for weeks if untreated.|
|Course||No new redness or blisters over time; condition confined to initial sites.||New lesions can appear over a week or more; different skin areas react at different rates.|
|"Signature" on skin||Reddish or brownish pigmentation, noticeable for months or years.||None.|
*Adapted from Sommer, Robert G. and Otis F. Jillson, 1967. "Phytophotodermatitis."
New England Journal of Medicine 276(26): 1484-6.