AOT40 is the Accumulated Concentration Over a Threshold of 40 ppb. If an hourly average ozone concentration exceeds 40 ppb the difference between the concentration and 40 ppb is added to a running total. For the critical levels the AOTs are summed during daylight hours over fixed time periods (Table below). To reduce the effect of inter-annual variability in ozone concentrations the critical level is assessed on the 5-year average AOT40 rather than an AOT40 for individual years. The AOT40 value is calculated as the concentration at canopy height. The suggested default canopy height for forest trees is 20 metres while for semi-natural vegetation it is calculated at a height of 0.2 metres (20cm).