By way of the detection system OPSIS, BP continually monitors the presence of hydrocarbons such as benzene around the PX unit. BP has agreed with the authorities to notify the 'Milieu-inspectie' (Environmental Inspectorate) and a number of public bodies the moment the system measures a value exceeding 100 microgram per cubic metre for more than 30 minutes. The amount of 100 micrograms per cubic metre is called the notification limit.