For purposes of this document, metalworking fluids (MWFs) are fluids used during machining and grinding to prolong the lift of the tool, carry away debris, and protect the surfaces of work pieces. These fluids reduce friction between the currint tool and the work surface, reduce wear and galling, protect surface characteristics, reduce surface adhesion or welding and carry away generated heat. Workers can be exposed to MWFs by inhaling aerosols (mists) and by skin contact with the fluid. Skin contact occurs by dipping the hands into the fluid, splashes, or handling workpieces coated with the fluids. The amount of mist generated (and the resulting level of exposure) depends on many factors: the type of MWF and its application process; the MWF temperature, the specific machining or grinding operation; the presence of splash guarding; and the effectiveness of the ventilation system in capturing and removing the mist. Substantial scientific evidence indicates that workers currently exposed to MWF aerosols have an increased risk of respiratory [lung] and skin diseases. These health effects vary based on the type of MWF, route of exposure, concentration, and length of exposure. To reduce the potential health risks associated with occupational exposures to metalworking fluids (MWFs), NIOSH recommends an exposure limit (REL) for MWF aerosol of penetrates below the larnyx in the respiratory system) as a time-weighted average (TWA) concentration for up to 10 hours per day during a 40-hour work week. Because the limited availability of thoracic samplers, measurement of total particulate mass is an acceptable substitute (see footnote 1 for details). The REL for total particulate mass is 0.5 mg/m1. The REL of 0.4 mg/m3 is based on four major considerations: the adverse respiratory health effects of MWF exposure; the selection of an index for measuring MWF aerosol exposure; the universal applicability of the REL to all types of MWFs; and, the technological feasibility of the REL. NIOSH also recommends the development and implementation of occupational safety and health programs, engineering controls, fluid management and medical monitoring to reduce MWF exposures. These recommendations are intended to prevent or greatly reduce respiratory disorders causally associated with MWF exposure. Whenever possible, reduce MWF aerosol levels below 0.4 mg/m3 (thoracic particulate mass) because some workers have developed work-related asthma or hypersensitivity pneumonitis at MWF exposures below the NIOSH recommended exposure level. It is also important to limit exposure levels based on the association between some past MWF exposures and various cancers and because the minimization of exposures by skin contact helps prevent allergic and irritant skin disorders.