Acetic acid is corrosive at concentrations above roughly 10%, and it becomes increasingly dangerous as concentration rises. The dilute form you know as household vinegar (about 5%) is a mild irritant at worst, but glacial acetic acid at 99% or higher can cause serious burns to skin, eyes, and the lining of your respiratory tract. The answer depends entirely on concentration.
How Concentration Changes the Risk
At 5% to 8%, acetic acid is ordinary vinegar. It can sting an open cut and irritate your eyes, but it won’t burn healthy skin on brief contact. NIOSH guidelines don’t require skin protection until concentrations exceed 10%, and emergency eyewash stations are recommended for workplaces handling anything above 5%.
Between 10% and 80%, the acid is classified as a transportation hazard and requires protective clothing to handle safely. Some vinegar products sold for cleaning or horticultural use range from 11% to 30% acidity, and at least one commercial product reaches 75%. These are not food-grade, and they carry real burn risk.
Above 80%, acetic acid enters a more serious hazard category. At roughly 99.5%, it’s called “glacial” acetic acid because it freezes into ice-like crystals at about 62°F. NOAA classifies glacial acetic acid as corrosive to both metals and tissue, with a health hazard rating of 3 out of 4, meaning it can cause serious or permanent injury.
What Concentrated Acetic Acid Does to Tissue
Like other acids, concentrated acetic acid reacts with water in your tissues and generates heat in the process. On the skin, this produces chemical burns that deepen the longer the acid stays in contact. In the eyes, it chars the surface layers of the cornea and causes collagen to shrink and coagulate, which can lead to permanent scarring or vision loss if not treated immediately.
Vapor exposure is also a concern. Breathing concentrated acetic acid fumes causes coughing, chest pain, and irritation of the nose and throat. At higher exposures, it can trigger nausea and vomiting. OSHA sets the permissible workplace exposure at 10 parts per million averaged over a work shift, with a short-term ceiling of 15 ppm. At 50 ppm, conditions are considered immediately dangerous to life or health. One particularly serious risk: lung swelling (pulmonary edema) can develop 24 to 48 hours after a heavy vapor exposure, well after the initial symptoms have passed.
What It Corrodes Beyond Skin
Acetic acid doesn’t just damage living tissue. Glacial acetic acid attacks many common metals and plastics. If you’re storing or handling concentrated solutions, material choice matters.
- Low-density polyethylene: Resistant at room temperature, but breaks down at 140°F (60°C).
- High-density polypropylene: Unsuitable at any temperature, even room temperature.
- Polycarbonate: Slowly attacked at room temperature, completely unsuitable when warm.
Glass and certain fluoropolymers hold up well, which is why laboratory-grade acetic acid typically ships in glass bottles. Standard metal containers, unless specifically lined, will corrode over time.
Glacial Acetic Acid Is Also Flammable
Beyond corrosivity, glacial acetic acid has a flash point of 104°F, which means it can ignite from a spark or flame at temperatures only slightly above a hot summer day. It earns a flammability rating of 2 out of 4, placing it in the “moderately heated before ignition” category. When it burns, it produces irritating vapors. This combination of corrosive and flammable properties is why labs and industrial facilities treat it with more caution than its vinegar origins might suggest.
What to Do if Concentrated Acetic Acid Contacts Your Skin or Eyes
For eye contact, flush immediately with cool water for at least 30 minutes, lifting your upper and lower eyelids periodically. Remove contact lenses while rinsing. This is a medical emergency that requires professional treatment right away.
For skin contact, remove contaminated clothing quickly and wash the affected area with large amounts of soap and water. Even if the burn looks minor, medical evaluation is warranted because acid burns can deepen over time.
For vapor inhalation, move to fresh air immediately. Because lung swelling can develop on a delay of one to two days, medical observation for 24 to 48 hours is recommended after significant exposure, even if you feel fine initially.
Household Vinegar Is a Different Story
Standard white vinegar at 5% acidity has a pH around 2.4 to 2.5, which is acidic but not corrosive to skin under normal use. You can clean with it, cook with it, and pickle with it without protective equipment. The risk begins when people purchase high-strength vinegar (20% or above) for weed control or heavy-duty cleaning without realizing it behaves much more like an industrial chemical than a kitchen staple. If the label says anything above 10% acidity, treat it with respect: wear gloves, protect your eyes, and keep it away from bare skin.