Black heads are caused if your skin is exposed to dirt and pollution. We can not stop going out in the sun or pollution to avoid black heads. Neither can you afford to stay at your home entire day. The best you can do to prevent these acnes and black heads is clean and scrub your skin periodically.
Out skin releases dead cells daily. These dead cells keep on accumulating over the new skin and block their way to get fresh air from outside. Also people expose themselves to pollution and the pores inside the skin are filled with dirt and dust. Washing your face will not get rid if this dust completely and as days will pass you will see formation of black heads on your face, neck or back.
Scrubbing is the best way to remove this dirt; it also removes the layer of dead cells from your skin and let the younger skin breathe properly. You must scrub your face at least twice a week or thrice if you over expose your self to pollution. Also you must use a body scrub once every week. Face is uncovered part of our body and is exposed maximum to dirt and sun. Try covering your face when going out in sun.