LEV stands for Local Exhaust Ventilation. It is an engineering control put in place to reduce or completely remove operator exposure to substances.
LEV is often the most effective way of removing potential operator exposure especially when the design of extraction is carried out at source.
LEV systems require routine testing and maintenance, dependant on what process they are used on, and the design of the system. As it is often the only engineering control in place, it is imperative that the system is always working as the design intended.
If you are able to obtain a copy of a datasheet for the product you are using, it may state that LEV is recommended or mandatory whilst using their product.