oil sludge in air hose?

so when i was changing my alternator i took the hose off that leads from the air filter and noticed thick oily sludge from the breather hose that comes out of the engine (19j) , the air filter is clean its just the hose that is minting, is there any way to prevent this or is it just a **** design and will have to keep replacing the hose and filter? cheers!!!

forgot to mention the sludge was in the air filter hose

sorry, cancel this post I didn't look at the previous one! silly me