Chapter 10: Acids and alkalis · Part 1: Alkalis
Sodium hydroxide
- Poison schedule (Australia)
- Contaminant risk
The worst fact: submissions to Australia's medicines regulator reported 4 child deaths in Queensland over 20 years, aged 1, 2, 3 and 10, at home and at work, from drain cleaners, oven cleaners and other caustic cleaners. The regulator's expert committee warns that "even brief oral exposure, without swallowing, of small volumes can result in severe injury."
Why brands use it: it dissolves fat and hair, so it clears drains and strips baked-on oven grease.
Take care if: you have children. Lock it away: high cupboards aren't enough, because children climb. Most child cases come from poor storage or while an adult is diluting the product. In Australia it's a Schedule 5 Poison at 5% or less when the pH is above 11.5, and a Schedule 6 Poison above 5%.
Comes from: salt water, split by electricity into caustic soda, chlorine and hydrogen.
Myth check: "It's safe in any pipe": no. It attacks aluminium, tin, lead and zinc, giving off flammable hydrogen, and it gets hot when it meets water.
Contaminants: mercury. Older factories made it using mercury, which can carry into the product. The EU banned that method from December 2017, and a global treaty set a 2025 phase-out, with some countries exempt until 2030. Australia has no such factory. Food-grade caustic soda in the EU must have no more than 1 mg/kg. Mercury is unlikely in a well-made product, but it's a factor you can't see on the label.
Fair to say: in 2026 Australia's regulator decided not to tighten its scheduling, finding that stricter rules were unlikely to prevent child poisonings and that drain and oven cleaners have few alternatives that work as well. These products carry strong warnings and usually come in child-resistant packs. Makers also add a little to bleach to keep it stable (see Chapter 4).
Correct as of November 2026. What the badges mean
Sources
- Australian Government. Poisons Standard October 2026 (SUSMP), F2026L01327: sodium hydroxide, potassium hydroxide, alkaline salts, monoethanolamine, sulfamic acid, hydrochloric acid, acetic acid; Appendix B (sodium bicarbonate).
- TGA. Notice of interim decisions (18 Dec 2025) and final decisions (25 May 2026): ACCS #40, sodium and potassium hydroxide.
- National Drugs and Poisons Schedule Committee. Record of Reasons, 46th meeting (2006: alkaline salt dishwasher detergents) and 58th meeting (2010: lye water).
- Safe Work Australia. Hazardous Chemical Information System (HCIS): classifications for each chemical, and for ethylene oxide, carbon monoxide and acetaldehyde.
- Better Health Channel (Victorian Department of Health). Poisoning and child safety; Mould removal at home.
- European Union. Regulation (EU) 2017/852 on mercury; Regulation (EU) 231/2012 food additive specifications. Minamata Convention on Mercury.
- ILO/WHO International Chemical Safety Cards: sodium hydroxide; sulfamic acid (0328).
- HSDB (US National Library of Medicine), via PubChem: production and impurity data for each chemical.
The full source list is at the end of Chapter 10.