The hydroxyl radical is often referred to as the "detergent" of the troposphere because it reacts with many pollutants, decomposing them through "cracking", often acting as the first step to their removal. It also has an important role in eliminating some greenhouse gases like methane and ozone, as well as inactivating pathogenic viruses and bacteria and neutralising allergenic pollens and mould spores. 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. For instance methane, which reacts relatively slowly with hydroxyl radical, has an average lifetime of >5 years and many CFCs have lifetimes of 50 years or more. Other pollutants, such as larger hydrocarbons, can have very short average lifetimes of less than a few hours.
Hydroxyl radicals can occasionally be produced as a byproduct of immune action. Macrophages and microglia most frequently generate this compound when exposed to very specific pathogens, such as certain bacteria. The destructive action of hydroxyl radicals has been implicated in several neurological autoimmune diseases such as HAND when immune cells become over-activated and toxic to neighboring healthy cells.
Hydroxyl radicals attack essential cell components and are therefore lethal to pathogenic viruses and bacteria (both gram -ve and +ve) – both in the air and on surfaces. Pathogenic viruses suffer from oxidation of their surface structures. Hydroxyl radicals disrupt the lipid envelope and/or capsid around the virus, causing lysing. They also penetrate the virus’s interior and disrupt the genome. These actions inactivate the virus. Hydroxyl radicals also pass through the outer cell wall structures of bacteria and oxidise the membrane responsible for electron transport, making the organism non-viable.
Hydroxyl radicals play a key role in the oxidative destruction of organic pollutant using a series of methodologies collectively known as advanced oxidation processes (AOPs). The destruction of pollutants in AOPs is based on the non-selective reaction of hydroxyl radicals on organic compounds. It is highly effective against a series of pollutants including pesticides, pharmaceutical compounds, dyes, etc.