Most notably hydroxyl radicals are produced from the decomposition of hydroperoxides (ROOH) or, in atmospheric chemistry, by the reaction of excited atomic oxygen with water.

Hydroxyl radicals are also produced during UV-light dissociation of H2O2 (suggested in 1879) and likely in Fenton chemistry, where trace amounts of reduced transition metals catalyze peroxide-mediated oxidations of organic compounds.

[3] The rate of reaction with the hydroxyl radical often determines how long many pollutants last in the atmosphere, if they do not undergo photolysis or are rained out.

Mechanisms for scavenging peroxyl radicals for the protection of cellular structures include endogenous antioxidants such as melatonin and glutathione, and dietary antioxidants such as mannitol and vitamin E.[6] The hydroxyl •HO radicals is one of the main chemical species controlling the oxidizing capacity of the Earth's atmosphere, having a major impact on the concentrations and distribution of greenhouse gases and pollutants.

The lifetime of •HO radicals in the Earth atmosphere is very short, therefore •HO concentrations in the air are very low and very sensitive techniques are required for its direct detection.

[9] Global average hydroxyl radical concentrations have been measured indirectly by analyzing methyl chloroform (CH3CCl3) present in the air.

The results obtained by Montzka et al. (2011)[10] shows that the interannual variability in •HO estimated from CH3CCl3 measurements is small, indicating that global •HO is generally well buffered against perturbations.

This small variability is consistent with measurements of methane and other trace gases primarily oxidized by •HO, as well as global photochemical model calculations.

Studies of •HO distribution in Taurus Molecular Cloud-1 (TMC-1)[20] suggest that in dense gas, •HO is mainly formed by dissociative recombination of H3O+.

Very high densities are required to thermalize the rotational transitions of •HO,[24] so it is difficult to detect far-infrared emission lines from a quiescent molecular cloud.

Nevertheless, HI observations have a fundamental difficulty when are directed to low mass regions of the hydrogen nucleus, as the center part of a diffuse cloud: Thermal width of hydrogen lines are the same order as the internal velocities' structures of interest, so clouds components of various temperatures and central velocities are indistinguishable in the spectrum.

CO has transitions in a region of the spectrum (wavelength < 3 mm) where there are not strong background continuum sources, but •HO has the 18 cm emission, line convenient for absorption observations.

[16] Observation studies provide the most sensitive means of detections of molecules with sub thermal excitation, and can give the opacity of the spectral line, which is a central issue to model the molecular region.

Studies based in the kinematic comparison of •HO and HI absorption lines from diffuse clouds are useful in determining their physical conditions, especially because heavier elements provide higher velocity resolution.

Application in water purification Hydroxyl radicals also play a key role in the oxidative destruction of organic pollutants.