Poison ivy rash vs heat rash?

The timing and the shape split them. Poison ivy is an allergic reaction to urushiol oil, and it follows a script: contact with the plant (hiking, yard work, a dog that ran through it), then 12 to 72 hours later, intensely itchy red patches and streaks, often in lines where a branch brushed the skin, frequently progressing to blisters, and lasting 1 to 3 weeks, per MedlinePlus. Heat rash is blocked sweat ducts: tiny pinprick bumps or small clear blisters appearing during or right after hot, sweaty conditions, concentrated where sweat is trapped (neck, chest, back, skin folds, under elastic), mildly prickly or stinging rather than furiously itchy, and gone within a few days of cooling and drying the skin. The streaks are the visual tell: linear streaks of rash scream poison ivy; heat rash never streaks. Two poison ivy clarifications people get wrong: the rash is not contagious and does not spread from person to person or from scratching (blister fluid does not spread it; only oil residue does, and once skin is washed the oil is gone, so "spreading" over days is just slower-reacting areas), but unwashed clothing, tools, or pet fur can re-expose you. Care: poison ivy needs cool compresses, calamine, oral antihistamines for the itch, and sometimes a steroid course from a clinician for widespread or facial involvement; heat rash needs cooling, loose clothing, and dry skin, nothing else. The see-a-doctor lines for poison ivy: face, eyes, or genitals involved, rash over a large body area, trouble breathing after burning poison ivy (inhaled smoke is dangerous), or signs of infection (pus, fever, spreading warmth); for heat rash, only if it crusts, oozes, or fevers, which suggests infection.