There are several developers that can cause contact
dermatitis. The most common is Metol but Hydroquinone can do
so as well. At one time Hydroquinone was used as part of a
method of dying furs. It was discovered that many people
would break out in a rash when wearing these furs and the
fur could touch the skin.
  Probably the most virulent developer is
paraphenylenediamine. PPD was popular as an extra-fine-grain
developer in the 1930's and 1940's but fell out of favor for
a number of reasons, its toxicity being one of them. Many
color developers are related to PPD. The newer ones are much
less toxic than PPD but all of these materials should be
handled with caution. All of these developing agents are
sensitizers. That means that one can develop a sensitivity
to them though use. The rash caused by developers can reach
a point where it is not trivial. Its recommended that you
wear nitrile gloves or use tongs where handling of film or
paper in the developer is necesary.