bro
how do u determine whether its dirty?
by using the fingers to determine?
lolz... use hand or eyes see lor... if dirty with sand and dirt means time to wash lor... vr easy only...lol....for me i'll use my eyes to determine lo kekekeke.
if u feel tat it is dirty then ma wash lo :D
terbalik n use lor... lolz...if upper part dirty, lower part no dirty & outer part dirty & inner part no dirty... then how...???
terbalik n use lor... lolz...
of coz wash lah.... sure on top dirty while bottom clean... if bottom also dirty means ur air filter not efficient in filtering dirt n dust...