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What is the 'Ozone and heat plan' in Belgium and what are the different phases?


In 2003, Europe was struck by an extraordinary heat wave. Because of the high temperatures and ozone concentrations there was a sharp increase in morbidity and mortality in risk groups (elderly people, people with respiratory troubles).

In response to this heat wave, several countries set up action plans for heat waves in order to better control crisis situations like that in 2003. The goal of these plans is to be able to react quickly, in order to reduce the health impact of high temperatures and ozone concentrations on risk groups by taking appropriate measures.

In Belgium, it was decided to set up a national plan with thresholds for both temperature and ozone. Since the summer of 2005, this 'Ozone and heat plan' (further referred to as 'the plan') has been in operation. The definition of heat wave in the current plan is not the same as the one used by climatologists.

The different phases of the plan

The plan consists of 3 phases, of which the second phase, the warning phase is further divided into two levels:

- a vigilant phase
- a warning phase, subdivided into levels 1 and 2
- an alarm phase

The warning and alarm phases kick in when a number of criteria are met. These criteria are based on results of 5-day weather forecasts, daily ozone measurements and 2-day ozone forecasts.

The first two phases of the plan, the vigilant and warning (level 1 and 2) phases are based on objective criteria (period, temperature and ozone concentration). The activation of the alarm phase, in contrast, requires additional assessment criteria ("the required actions for this phase are taken when the threshold has been reached and the measures already taken have to be intensified").

Before the alarm phase is activated, according to the original 2004 plan, a risk assessment group (RAG) is established. This RAG evaluates whether it is opportune to activate the alarm phase and if additional measures are needed. The suggestions and decisions of the RAG are then delivered to the risk management group (RMG) that is composed of representatives of the relevant authorities. The RMG eventually decides whether or not the alarm phase is activated. The heat and ozone criteria for the alarm phase were first reached in August 2020, the RAG and RMG were established and the alert phase declared on 8 August 2020.

The three possible phases of the plan are:

1. The vigilance phase

The vigilance phase starts on the 15th of May each year and ends on the 30th of September. This period is fixed and is therefore independent of weather conditions or air quality. From the 15th of May onwards, the actions are started systematically. As such, it is not dependent on a threshold, but on a period.

2. The warning phase

(Note: the criteria to trigger the warning phase were changed in 2017.)

The warning phase is triggered when the Tcumul is higher or equal to 17°C.

To calculate the Tcumul, the forecasted maximum temperatures in Uccle are used. The Tcumul is the sum of the difference between the 'maximum predicted temperature' and 25°C in Uccle for the following five days (D+1 to D+5). Here, only the positive differences are taken into account.

The warning phase ends when the Tcumul is below 17°C (on day D0) AND the predicted maximum temperature in Uccle on day D+1 is below 25°C.

3. The alarm phase

(Note: the criteria to trigger the alarm phase were changed in 2019.)

The alarm phase is triggered when:

the criteria for the warning phase are met


a maximum temperature higher or equal to 28°C is forecast for the current day


an exceedance of the European ozone information threshold of 180 μg/m³ was measured at at least one measurement site in Belgium the previous day


for the current day, exceedances of the European ozone information threshold are predicted in a significant part of the country.


When the RMG decides to activate the alarm phase, the measures already taken are intensified.


SOURCE: Coordination protocol for the implementation of the "ozone and heat plan" between the Flemish, Walloon and Brussels Capital Regions and IRCEL.