Help! Dusty Apartment

Ok,

To begin, I live in a tiny, 500 sq ft apartment on the second floor of a multi-unit home. Since I am not worried about security, I leave my windows cracked open almost 24 hours a day to allow air to circulate; not too wide, between 1-5 inches. I noticed that even when I apply the principle of cleaning top to bottom, my apartment gets pretty dusty. Is this due to the windows being open? Is this normal for other people? Am I just trippin? HELPPPPP