Smog Plan: peak in fine-particle pollution

Pollution peaks are large and rapid increases in the concentrations of pollutants in the air.  Fine-particle pollution peaks occur in particular in winter or at the beginning of spring, often under particular meteorological conditions (temperature inversions), which prevent the dispersion of atmospheric pollutants.

Combined with the unfavourable dispersion conditions in the atmosphere, ammonia emissions can also play a large role in the formation of fine particles, especially in the spring. Ammonia comes from agricultural landings as well as from livestock. This compound can react with nitrogen oxides (first transformed into nitric acid), mainly from traffic, to form ammonium nitrate, and causes a significant increase in fine particle concentrations.

The main sources of emissions of these pollutants are the heating of buildings, transport, industrial activity, and agriculture.

The term "smog" is the contraction of two English terms, "smoke" and "fog". "Smog" refers to the simultaneous presence of these two components in the air.

An action plan in the event of a peak in fine-dust pollution was adopted by the Walloon Government on 17 July 2008. The Walloon Air and Climate Agency (AwAC) manages these alerts, in collaboration with the Regional Crisis Centre (CRC) and the Interregional Environment Unit (CELINE). A memorandum of understanding between the country's three regions entrusted CELINE with forecasting and monitoring pollution throughout the duration of the period and also delegated the initiation of the phases linked to the period to CELINE. The objective is to harmonise the implementation of emission reduction measures by ensuring simultaneous activation of the different phases encountered during a pollution episode in each Region.

In Wallonia, three peak particle thresholds are set out:

  • an information threshold (threshold 0) of 50 µg/m³ on average over 24 hours for PM10 and 35 µg/m³ on average over 24 hours for PM2.5. The information phase is activated when the threshold is reached and forecasts indicate that the exceedance should last at least 24 hours.
  • an alert threshold (threshold 1) of 70 µg/m3 on average over 24 hours for PM10 and 50 µg m³ on average over 24 hours for PM2.5. The alert level is activated when the CELINE unit anticipates that this threshold will be exceeded for two consecutive days.

  • a second alert threshold (threshold 2) at 150 µg/m³ on average over 24 hours for PM10. The enhanced alert level is activated when the CELINE unit anticipates that this second threshold will be exceeded for two consecutive days

Exceeding the first threshold involves providing information and recommendations for the population.

When the alert threshold is reached, restrictive measures are also put in place. These actions concern the limitation of vehicle speed on motorways and expressways, accompanied by the strengthening of controls. The companies most concerned by fine-dust emissions are also called on to take measures to temporarily reduce their emissions.

In addition, the municipalities in the most exposed areas (Charleroi, Engis, Liège and Tournai) have adopted a specific plan based on three pillars: reducing the speed of vehicles in town, raising awareness among citizens, and reducing the temperature in the city's public buildings.

Information when one of these thresholds is exceeded is provided mainly via the media, the internet, sending a newsletter to people registered on the CELINE site, or automated text messages and emails to people registered for the free service offered on this site (see user area).

In the event of a pollution peak, it is recommended that sensitive individuals do not engage in intense and/or prolonged physical exercise (such as jogging, for example). If necessary, do not hesitate to seek medical advice.

Link : Pollution peak.pdf

The heat wave and ozone peak plan

In the summer of 2003, Europe was hit by a heat wave of exceptional magnitude. This scorching episode was characterised by exceptional temperature and ozone peaks, and a sharp increase in morbidity and mortality, more markedly in groups identified as "at risk".

In addition, the limits displayed by the public authorities of certain states to curb the consequences of this episode have led to strong discontent in public opinion.
Following this episode, many states therefore undertook to set up or strengthen their crisis management structures in connection with this type of event via heatwave or heat wave management plans.

The purpose of these plans is to anticipate the occurrence of heat waves, and to define the measures to be taken to prevent and limit their effects on health and social well-being, all while paying very particular attention to at-risk populations.
The Belgian national plan, “Heat wave and ozone peaks Plan”, dates back to 2005. The aim of this plan was to anticipate the appearance of heat and ozone peaks.

It aimed to proactively define measures intended to prevent and limit the impacts of the ozone and heat on health. In this regard, special attention is paid to at-risk groups.

In Belgium, it was decided to combine the management of high temperatures with the issues of ozone peaks, the two often being linked. From 2015, it was decided to rename the plan "High heat and ozone peaks plan".

Until 2014 inclusive, the execution of the high heat and ozone peaks plan was the responsibility of the Federal State. Under the sixth state reform, it is now the federated entities that are responsible for implementing the plan (awareness, communication, etc.). However, the alert phase falls under federal authority.


Phases of the plan :

The “high Heat and ozone peaks Plan” is made up of 3 phases, with its second phase, the warning phase, subdivided into two levels:

  1. a vigilance phase;
  2. a warning phase;
  3. an alert phase

The transition from one phase to the other depends on whether the thresholds are reached or not. The parameters that make up these thresholds are based on the results of the 5-day weather forecast, daily ozone measurements, and 2-day ozone forecast.
The first two phases of the plan, vigilance and warning, are based on objective criteria (period, temperature and ozone concentration) while the alert phase requires more than judgment criteria ("...when the threshold has been reached and it transpires that the measures already taken must be scaled up").
In order to make this judgment for the alert phase, the initial 2004 plan is to bring together a risk assessment unit (Risk Assessment Group, RAG), which will decide whether additional measures are necessary and assess if triggering the alert phase is necessary.

These decision and assessment proposals must then go to a risk management unit (Risk Management Group, RMG) comprising the competent authorities. Since the alert phase has never been reached in 10 years, these two units have never been set up within the framework of this plan.

1. Vigilance phase
Operational period for implementing the plan from 15 May to 30 September each year, regardless of weather conditions. It is active when the temperature forecasts and ozone concentrations are below the thresholds determined for the warning and alert phases.

2. Warning phase

The criteria for activating the warning phase were modified in 2017.The warning phase is triggered when Tcumul is greater than or equal to 17°C (note: the temperature values forecast at Uccle are used).

Tcumul is obtained by finding the sum of the differences between the values of "maximum temperatures forecast" and the threshold of 25°C for the next five days (D+1 to D+5). Only positive differences are taken into account.

The warning phase ends when Tcumul is less than 17°C (on day D0) and the maximum temperature forecast in Uccle on day D+1 is less than 25°C.

3. Alert phase

The criteria for the alert phase were modified in 2019 and to be activated, the conditions below must all be met: - the temperature criterion for the warning phase is satisfied;

- the maximum temperature forecast for the same day is greater than or equal to 28°C;

- there has been an overnight measurement, at at least one measuring point, of an hourly average ozone concentration greater than 180 μg/m³ (European Information threshold);

- the hourly average ozone concentrations for the same day are forecast to be greater than 180 μg/m³ over a significant part of the country;

- it transpires that the measures already taken must be scaled up.

Source: Coordination protocol between the 3 Regions and CELINE within the framework of the implementation of the "High heat and ozone peaks" plan


As for the ozone peak, it is defined as follows:

An ozone peak occurs when the ozone concentration exceeds a certain value. The European Union has issued directives which define target values for the protection of the population. The European threshold value for public information was set at an average hourly concentration of 180 micrograms of ozone per cubic metre of air. The alert threshold is exceeded once the hourly average concentration reaches 240 micrograms of ozone per cubic metre of air.

The effects of a heat wave are sunstroke, heat stroke, heat cramps, exhaustion, etc. The ozone peak can cause eye and throat irritation, shortness of breath, and headaches. However, some very simple measures can help prevent these disorders:

  • Stay hydrated by drinking more than usual and, in particular, do not wait until you are thirsty. Sugary drinks and alcohol are not recommended.

  • Protect yourself from the sun: hat, sunscreen, light clothing, etc.

  • Stay indoors during the hottest hours, preferably in a cool place. Close curtains and windows during the hottest hours


  • Limit physical efforts and travel


  • Cool off: shower, pool, etc.

  • ...

The most vulnerable people are young children, the elderly, people who are socially isolated, and those who engaging in intense exertion. However, the harmful effects can also affect healthy people.

The "Ozone and heat wave" leaflet is available here and for more information: