Millipedes generally have little impact on human economic or social well-being, especially in comparison with insects, although locally they can be a nuisance or agricultural pest. Millipedes do not bite, and their defensive secretions are mostly harmless to humans — usually causing only minor discoloration on the skin — but the secretions of some tropical species may cause pain, itching, local erythema, edema, blisters, eczema, and occasionally cracked skin. Eye exposures to these secretions cause general irritation and potentially more severe effects such as conjunctivitis and keratitis. This is called a millipede burn. First aid consists of flushing the area thoroughly with water; further treatment is aimed at relieving the local effects.