How do you clean the E20739 model of Lasko Ionizing Tower Fan? Huge dust bunnies inside fan remain even when vacuumed outer surface with a brush attachment. There are two screws at the top - four screws at the bottom on the back cover of the fan, but they cannot be moved with a screw driver. Are they supposed to be removed to clean the fan? I am experiencing very severe asthma and need to clean the product ASAP. Please help.