it may be dust or pollen, stired up by the air
conditioning. if you check/clean any filters the air
conditioning system may have, it could help.
if you increase the frequency of dusting, there will be less dust to be circulated in the air make you ill.
there is a chance that even if you have not had hayfever previously, it is possible to develop it, especially now what-ever pollen is in the air, is being well circulated.