Although an intimate mixture of a flammable dust and air may burn with explosive violence, not all mixtures will do so. There is a range of concentrations (explosive concentrations) of the dust and air within which the mixture can explode. The lowest concentration of dust capable of exploding is referred to as the Lower Explosive Limit (LEL). The lower explosive limit, sometimes called Minimum Explosible Concentration (MEC) of many materials can be be measured. LEL or MEC vary from 10 grams per cubic metre to about 500 grams per cubic metre. The upper explosive limits are not well defined and have poor repeatability under laboratory test conditions. The most violent explosions are produced when the proportion of oxygen present is not far removed from that which will result in complete combustion. The range of the explosive concentrations of a dust cloud is not simply a function of the chemical composition of the dust; the limits vary strongly with humidity and with the size and shape of the particles in the dust cloud. It depends on the circumstances in your factory.