This indicator shows the value of the AOT40 parameter, which reflects the effects of ozone on vegetation between May and July when open air ozone concentrations are at their highest.

The AOT40 value (expressed in [μg/m³] x hours) is defined as the sum of the differences between 1-hour concentrations above 80 μg/m³ (= 40 parts per billion [ppb]) and 80 μg/m³ within a given time period, using only hourly values measured each day between 8:00 a.m. and 8:00 p.m. CET.

In the period 2007–2014, the target value of the AOT40 parameter for plant protection (a five-year average for four preceding years plus the current year) was exceeded at all representative measuring sites except at Sv. Mohor nad Brestanico between 2009 and 2012.

The highest mean values were measured at the Otlica and Krvavec measuring sites. The Krvavec measuring site is located at a high-altitude. At this site, daily fluctuations of ozone concentration are relatively low, resulting in a higher sum of morning and afternoon concentrations. The Otlica measuring site is located slightly less than 1000 m above the sea level. High concentrations occur due to the transport of ozone from the valley and from Italy. Ozone concentrations and, consequently, the values of the AOT40 parameter, are highly dependant on the weather; the highest values occur during the years with long periods of dry, hot weather and abundant solar radiation.