 Is tea tree oil toxic? Anecdotal evidence suggests that the topical use of the oil is relatively safe and that adverse events are minor, self-limiting, and occasional. Published data, however, adds some caveats. It can be toxic if ingested. It can also cause skin irritation at higher concentrations. Now normally, tea tree oil reduces skin inflammation. 27 volunteers had histamine injected in their skin, the equivalent of like getting bitten by a fire ant. But the big red swollen mark significantly decreased after the application of tea tree oil. Here's where the bump was at 20 minutes after injection. Apply a placebo oil and it continues to get worse, before finally beginning to calm down. But if at 20 minutes you apply half of a single drop of pure tea tree oil, it stops the inflammation in its tracks and immediately starts getting better. Some people are sensitive to it though and it can instead trigger a rash. This is relatively rare though, with only about 1% of older children and adults having such a reaction, and none of the 40 younger children tested had a reaction, which is good since it can be found like 5% of diaper wipes and lotions. Most reactions, when they do occur, are caused by the application of pure oil, and so there are recommendations to keep the concentration of tea tree oil products applied to the skin under 1%. Moreover, manufacturers may want to use anti-oxidants and dark bottles to minimize exposure to light, since aged oxidized oils are more likely to induce allergic reactions. Hundreds of different compounds have been identified in tea tree oil, but the composition changes when exposed to light, air, humidity, and higher temperatures. It can start turning a green-brownish color, the viscosity changes, and the smell becomes turpentine like all bad signs. Even fresh tea tree oil shouldn't be ingested though. Two hours before arriving at the Pediatric Critical Care Unit, a four-year-old's mother had given him approximately two teaspoons of tea tree oil. Within 30 minutes, he had trouble walking, and shortly thereafter fell into a coma. They note that tea tree oil was in a bottle without a child-proof cap, but it didn't matter in this case because the cap wasn't mother-proof either. Similar cases are reported, even less than two teaspoons, though the reported adult poisoning cases have tended to involve larger doses. Thankfully, no human deaths caused by tea tree oil have been reported, though note the qualifier human. It has been implicated in the deaths of pets when used inappropriately, when trying to treat fleas or something. Pets in particular at risk because they're a habit of licking their fur. In humans, though, it can apparently be used safely if you avoid ingestion, apply only diluted oil topically, and only use oil that's been stored correctly.