In the internal combustion engine the three factors work together effectively: petrol, air/oxygen and the ignition spark produce an explosion inside the enclosed cylinder. For this to take place, the ratio of petrol to air must be correct. If the petrol tank is empty, the air filter blocked or if the ignition does not work, one of the components for triggering this explosion is missing and the motor will not start. Combustible materials mixed with air have a lower and an upper flammable limit and the explosive range lies between these limits. When considering the safety of the workplace, the lower flammable limit is the more important value. In many cases, a possible concentration ≤ 10 % of this value is considered to be safe.