But isn’t eczema caused by an allergy to something?

It depends what you mean by ‘allergy’. The strictly scientific
definition of allergy refers to ‘when a substance causes an
abnormally excessive response to arise from the body’s immune
or defence system’. This may be measured by determining the levels
of antibody in the white blood cells (lymphocytes). The allergic
reaction should be reproducible by ‘re-challenging’ with the same
substance – the same reaction will be produced each time the
substance is used.
Many people, however, use the word ‘allergy’ in a different way.
They may use it to imply that a certain disorder is caused by a
specific substance and that this disorder will disappear if the
offending agent is avoided. Unfortunately, this is not the case withatopic eczema. It is perhaps best imagined as a built-in reaction
that can be modified (but not caused) by the environment. Many
things in the environment can make eczema worse (e.g. woollen
clothes, dog hair), but this may be because they act purely as an
irritant rather than as a true allergen.
My GP says that my hand eczema is caused by things I
come in contact with. What does this really mean?
Contact is the most common cause of hand eczema in adults. Your
GP’s comment can be looked at in a couple of ways. First, some
substances are potentially irritant to the skin such that anyone
would eventually develop a rash that looked like eczema if they
were exposed to high enough concentrations for long enough. An
example here is strong detergent. Your problem could be that you
have a much lower threshold to a wide range of possible irritants,
so you react to lower concentrations and a shorter contact time.
This will especially be the case if you have a little bit of eczema or
just dry skin at the time of the contact. The same contact on an
area of normal skin would not cause you a problem.
The other possibility is that you are allergic to a particular
substance (or several if you are unlucky). This substance would
be unlikely to cause any reaction in someone without an allergy to
it no matter how long they were in contact with it, and it would be
able to cause eczema on any part of your skin after the initial
‘sensitising’ reaction had taken place. Your first contact that caused
a problem would lead to eczema after a few days or so, but
subsequent contacts would cause the eczema to start within a
matter of hours. An example of this is an allergy to something in a
perfume such as ‘balsam of Peru’.