Smog is expected to cause “poor” air quality in parts of central and southern Netherlands on Wednesday and Thursday amid extreme heat, the National Institute for Public Health and the Environment (RIVM) reports.

On Wednesday, the highest risk is expected in parts of Flevoland, Utrecht, and the central and western areas of Noord-Brabant. On Thursday, smog is expected south of a line running from Leiden through Amersfoort to Apeldoorn. According to the institute, the smog results from the buildup of air pollution on sunny days with little wind.

The agency warned that smog can lead to increased respiratory problems such as coughing and shortness of breath, worsening asthma symptoms, and reduced lung function.

It advised people sensitive to smog to stay indoors and limit heavy physical exertion, particularly in the afternoon and early evening when air pollution is expected to be highest. Children, older adults, and people with asthma are considered most vulnerable.

From Wednesday to Friday, the KNMI will set a code orange weather warning for extreme heat in the central and southern parts of the country. A code yellow will be Groningen, Friesland, and Drenthe.

Rijkswaterstaat said its Heat Plan, already in effect in Limburg and Noord-Brabant, will be expanded Wednesday morning to Gelderland, Utrecht, Zeeland, and Zuid-Holland as road surface temperatures are expected to rise. Under the plan, stranded motorists are moved quickly to nearby facilities such as gas stations or parking areas to avoid prolonged exposure to the heat.

Authorities urge people to take precautions during hot weather and to look out for vulnerable people, as well as animals and nature.