A lot of bacteria grows in the range of 40-100F (i.e. room temperature). It's definitely not recommended to defrost meat at room temperature. In fact, you are not supposed to leave meat at room temperature for more than an hour.
However, defrosting in the refrigerator can take a long time and require you to plan at least one day ahead of time. I'm not so good at this, which leads to a safe and fast solution: Defrost meat in a waterproof ziploc bag in cold water. Change the water every 30 minutes until defrosted. The water is a better conductor of heat than air, so the defrosting is quite fast and the water is cold so there's minimal safety risk.
From the USDA:
Uh, oh! You're home and forgot to
defrost something for dinner. You grab
a package of meat or chicken and use
hot water to thaw it fast. But is this
safe? What if you remembered to take
food out of the freezer, but forgot
and left the package on the counter
all day while you were at work?
Neither of these situations are safe,
and these methods of thawing lead to
foodborne illness. Food must be kept
at a safe temperature during "the big
thaw." Foods are safe indefinitely
while frozen. However, as soon as food
begins to defrost and become warmer
than 40 °F, any bacteria that may have
been present before freezing can begin
Foods should never be thawed or even
stored on the counter, or defrosted in
hot water. Food left above 40 °F
(unrefrigerated) is not at a safe
Even though the center of the package
may still be frozen as it thaws on the
counter, the outer layer of the food
is in the "Danger Zone," between 40
and 140 °F – at temperatures where
bacteria multiply rapidly.
When defrosting frozen foods, it's
best to plan ahead and thaw food in
the refrigerator where food will
remain at a safe, constant temperature
– 40 °F or below.
There are three safe ways to defrost
food: in the refrigerator, in cold
water, and in the microwave.
Refrigerator Thawing Planning ahead is
the key to this method because of the
lengthy time involved. A large frozen
item like a turkey requires at least a
day (24 hours) for every 5 pounds of
weight. Even small amounts of frozen
food -- such as a pound of ground meat
or boneless chicken breasts -- require
a full day to thaw. When thawing foods
in the refrigerator, there are several
variables to take into account. Some
areas of an appliance may keep the
food colder than other areas. Food
placed in the coldest part will
require longer defrosting time. Food
takes longer to thaw in a refrigerator
set at 35 °F than one set at 40 °F.
After thawing in the refrigerator,
ground meat and poultry should remain
useable for an additional day or two
before cooking; red meat, 3 to 5 days.
Foods defrosted in the refrigerator
can be refrozen without cooking,
although there may be some loss of
Cold Water Thawing
This method is faster than
refrigerator thawing but requires more
attention. The food must be in a
leak-proof package or plastic bag. If
the bag leaks, bacteria from the air
or surrounding environment could be
introduced into the food. Also, meat
tissue can also absorb water like a
sponge, resulting in a watery product.
The bag should be submerged in cold
tap water, changing the water every 30
minutes so it continues to thaw. Small
packages of meat or poultry – about a
pound – may defrost in an hour or
less. A 3- to 4-pound package may take
2 to 3 hours. For whole turkeys,
estimate about 30 minutes per pound.
If thawed completely, the food must be
Foods thawed by the cold water method
should be cooked before refreezing.