Group of keyboard keys not working.

The following keys have stopped working on my "oldish" (10 years) keyboard. They are the 5, 6, 7 an 8 keys running along the top of the keyboard, and the f5, f6 and f7 keys just above the 5, 6, 7 and 8 keys. In other words, the inoperative keys are grouped together. No liquids have been spilt on the keyboard, but it has been in a dusty atmosphere. Could dust have got in the keys? Are they easy to get off and clean? The keyboard is connected to a 27 " iMac which is about 10 years old, as is the keyboard.