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 ozone excess for forests is calculated in the same way as for vegetation, the only difference being that a longer period is taken into account, namely from April to September. The current Air Quality Directive does not include any objectives for the ozone excess for forests. These indicators quantify only ozone exposure, i.e. not the effective ozone uptake by (and therefore damage caused to) vegetation.