For each pollutant measured an index is calculated which is related to one of four bands, Low, Moderate, High, Very High.

This page lists the affects of health of each of the pollution bands. People with particular health porblems may suffer stronger effects than indicated.

Banding	Index	Health Descriptor
Low	1, 2, or 3	Effects are unlikely to be noticed even by individuals who know they are sensitive to air pollutants
Moderate	4, 5, or 6	Mild effects, unlikely to require action, may be noticed amongst sensitive individuals.
High	7, 8, or 9	Significant effects may be noticed by sensitive individuals and action to avoid or reduce these effects may be needed (e.g. reducing exposure by spending less time in polluted areas outdoors). Asthmatics will find that their 'reliever' inhaler is likely to reverse the effects on the lung.
Very High	10	The effects on sensitive individuals described for 'High' levels of pollution may worsen.

Source: UK National Air Quality Archive

Find out more about the effects of individual pollutants.