What does the law have to say about this?
In Germany, it is mandatory to have a roof ventilation unit if you are transporting hazardous goods. In the Netherlands, there are currently no such regulations which is crazy, of course, as hazardous products such as gas cylinders, flammable liquids and chemicals can create highly dangerous situations. Sufficient ventilation is necessary in order to prevent the formation of explosive gas mixtures.
However, regulations relating to the transportation of hazardous substances in vans can be found in the Regulation on the land transport of dangerous substances (ADR) [Dutch: VLG]. If you carry limited quantities of hazardous substances in your van, the so-called 1000 points rule will apply. It specifies that sufficient ventilation must be provided in a van if you are carrying certain gases. This can be done by installing a roof ventilation unit.