Bequerels; I suspect this will be 320 Bequerels per cubic metre, meaning
that each cubic metre of air contains 320 radon atoms decaying per second.
(In general 1 Bequerel just means a quantity of any radioactive material in
which one atom decays per second).

Others will advise you on action, but I'd comment that it's not very much
above the action limit. There's a tendency for people to regard government
"limits" for all sorts of pollutents as "danger limits", e.g. 199 is "safe"
and 201 is an immediate threat to life and limb. In fact the limits are
usually levels where the risk is very small compared to other natural risks.