A pale blue, reactive gas with a pungent odour. At ground level, it forms through chemical reactions involving sunlight, volatile organic compounds, and nitrogen oxides (the main source being from vehicle emissions). Ozone also occurs naturally in the upper atmosphere and can move downward, contributing to ground level concentrations. Health risks include respiratory and cardiovascular diseases. Measured concentrations are reported in micrograms per cubic metre (µg/m3) of air.