Unusually reactive forms of oxygen

Some forms of oxygen, called free radicals, are unusually reactive, meaning
they react readily with other chemicals. These oxygens can damage DNA
directly (by causing strand breaks) or can convert bases into new unwanted
chemicals that, like most other chemical mutagens, then cause mispairing
during replication. Free radicals of oxygen occur normally in your body as a
product of metabolism, but most of the time, they don’t cause any problems.
Certain activities, such as cigarette smoking and high exposure to radiation,
pollution, and weed killers, increase the number of free radicals in your
system to dangerous levels.