Tetrahydrofuran (THF) is a contaminant of exposure and appears in human biofluids. The Food and Drug Administration (FDA) has announced recommendations to revise the permitted daily exposures for THF, and so has the International Conference on Harmonisation. THF oxidizes readily, which can lead to instability and result in cytotoxicity. It is a colourless low-viscosity liquid with a smell similar to diethyl ether and is one of the most polar ethers. Tetrahydrofuran is essentially considered in occupational toxicology, and acute poisoning cases are extremely rare. THF is often used for hydroborations used to synthesize primary alcohols. THF is often used as a solvent for Grignard reagents because of the oxygen atom's ability to coordinate to the magnesium ion component of the Grignard reagent (an organometallic chemical reaction involving alkyl- or aryl-magnesium halides). THF is often used in polymer science. For example, it can be used to dissolve rubber prior to determining its molecular mass using gel permeation chromatography. THF tends to form peroxides on storage in air.