pH measures how acidic or alkaline water is, and it affects the toxicity of many substances in the water as well as the ability of fish and invertebrates to breathe, reproduce, and survive. Outside the acceptable range, fish experience increasing physiological stress, and sensitive invertebrates like mayflies can be affected at levels that seem only mildly acidic. The UK's Water Framework Directive sets a freshwater pH range of 6 to 9 to protect aquatic life.