Acids and alkalis
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The strongest cleaning chemistry available, the most hazardous, and the place where several regulatory thresholds sit.
Two ends of the same axis
Acids dissolve mineral soil: limescale, rust, hard water marks, cement.
Alkalis dissolve organic soil: grease, fats, protein, burnt-on food.
Which is why a bathroom cleaner and a kitchen degreaser are at opposite ends of the pH scale and why neither works well at the other's job.
The acid side
Citric and lactic acid, mild, food-derived, and the usual choice for a consumer descaler.
Acetic acid, which is vinegar at low concentration and a different proposition at high.
Sulfamic and phosphoric acid, stronger, used in commercial descalers.
Hydrochloric acid, very strong, used in toilet and masonry products, and genuinely dangerous.
Sulfuric acid, mostly in drain products, and child-resistant packaging is required at 10 percent or more.
Acids attack metals, grout, natural stone and some surfaces the customer will apply them to, which is a labeling problem as much as a formulation one.
The alkaline side
Sodium carbonate and sodium metasilicate, moderate, good general degreasers.
Sodium hydroxide, very strong, used in oven and drain products.
Alkalis damage aluminum, soft metals and some finishes, and a strong one causes serious burns that are worse than they feel at first because there is no immediate pain.
Where the thresholds are
This is the paragraph to carry away, because the numbers are low.
Sodium or potassium hydroxide at 2 percent or more by weight in a liquid product requires child-resistant packaging. Ten percent or more in a dry product.
Sulfuric acid at 10 percent or more.
And well below either, a hazardous substance in a household product triggers cautionary labeling under the Federal Hazardous Substances Act: signal word, hazard, precautions, first aid, keep out of reach of children.
Two percent caustic is a small addition to lift grease and it is the point at which the closure becomes a legal requirement. Calculate it in the finished product, not in the raw material.