For vegetation, long-term exposure to ozone is more relevant than acute exposure. It is difficult to quantify the potential damage to plants caused by ozone pollution. In fact, ozone causes the most damage when it is taken up by the plant. The ozone concentrations in the atmosphere are, however, not a direct measure of ozone uptake in the plant. Many other parameters play a role: humidity level, soil condition, plant growth phase, etc. The European Union has defined an excess indicator for the protection of vegetation. The AOT40 for vegetation is the accumulated excess of hourly ozone concentrations above 80 μg/m³ between 8:00 and 20:00 CET (Central European Time = Universal Time (UT) + 1) in the months of May, June, July (=growth season). This indicator is designed for the protection of crops and (semi)natural vegetation. These indicators quantify only ozone exposure, i.e. not the effective ozone uptake by (and therefore damage caused to) vegetation.