Several studies indicate that aerobes can survive in the presence
of oxygen only by virtue of an elaborate system of defenses. Without
these defenses, key enzyme systems in the organisms fail to function
and the organisms die.
http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mmed.section.1029

Most symptoms of oxygen toxicity can be categorized into either
pulmonary (coughing, substernal soreness, and pulmonary edema) or
central nervous system (including body soreness, nausea, muscular
twitching, and convulsions).
http://www.osha.gov/SLTC/commercialdiving/more.html#toxicity

Present-day anaerobes are presumably the descendants of those primitive
organisms that followed the evolutionary path of "adapting" to rising
atmospheric O2 levels by restricting themselves to environments that
O2 did not penetrate. However, other organisms began the evolutionary
process of evolving antioxidant defense systems to protect against O2
toxicity. ... O2 was probably the first environmental air pollutant
to appear in large quantities on the planet.

However, even present-day aerobes suffer oxidative damage if they are
exposed to O2 at concentrations greater than 21% (2). Oxygen toxicity
has been demonstrated in plants, animals and microorganisms. For
example, exposure of adult humans to pure O2 at 1 atm pressure for
as little as 6 hr causes chest soreness, cough, and sore throat in
some subjects; and longer periods of exposure lead to lung damage. The
incidence of ocular damage in babies known as retrolental fibroplasia
("formation of fibrous tissue behind the lens") increased abruptly in
the early 1940s among babies born prematurely and led to many cases of
blindness. Not until 1954 was it realized that retrolental fibroplasia
is associated with the use of high O2 concentrations in incubators for
premature babies.
http://ehp.niehs.nih.gov/members/1994/Suppl-10/haliwell-full.html

Convulsions... pulmonary edema... lung damage... blindness.  Holy sh.t,
that stuff is more dangerous than latex!  For your own health and
safety, stop breathing now!