Cleanroom protective clothing is made of low-shedding materials to limit contamination. They also limit contamination caused by worker's clothing, hair, and skin. ISO-rated cleanroom clothing is low-shedding to prevent particles from the garment from contaminating the environment. Cleanroom garments are assigned a cleanroom class to indicate which cleanrooms they can be used in. Lower classes (ISO 1) indicate a cleaner garment than higher numbers (ISO 9). Coveralls provide continuous coverage from neck to ankle and also cover arms. Hoods cover the head, neck, and shoulders. Lab coats and frocks have front closures to cover the front, back, and arms. Shoe and boot covers prevent contaminants from footwear from dirtying surfaces and protect footwear from incidental contact with substances. Bouffants keep hair in place and veils cover the nose and mouth. They are often worn with hoods that cover the head and neck.