Environmental chemistry
Concentration and pH decide whether a substance in water is harmless or harmful, and each pH step is a factor of ten.
A worked example
20 mg of nitrate dissolved in 4 L of water is 20 / 4 = 5 mg/L, and a fall from pH 7 to pH 4 is three steps, so 1 000 times more acidic.
The mistake people actually make
The pH scale runs backwards from what you expect: a lower number means more acidic, so pH 3 is far stronger than pH 6. A substance can be harmless in traces and still build up to dangerous levels further along a food chain.