For maximum effectiveness and longest element life, the windshield and wiper blades should be kept clean, as dirt, tree sap, and road tar can cause streaking, smearing, and blade deterioration. It is advisable to wash the windshield with a commercial glass cleaner at least once a month and to wipe off the rubber blades with a wet rag afterward. Avoid moving the wipers back and forth by hand to prevent damage to the motor and drive mechanism. If the blades are cracked, broken, or torn, they should be replaced immediately, with replacement intervals varying based on usage; ozone deterioration typically limits blade life to about one year. If the wiper pattern is smeared or streaked, or if the blade chatters across the glass, replacement is necessary, and it is easiest to replace them in pairs. There are three types of wiper blade refills, differing in their replacement methods. One type features two release buttons located about one-third of the way up from the ends of the blade frame; pushing these buttons down releases a lock, allowing the rubber blade to be removed and replaced. The second type has two metal tabs that unlock when squeezed together, enabling the rubber blade to be withdrawn from the frame jaws, with a new one installed by inserting it into the front frame jaws and sliding it rearward to engage the remaining jaws. The third type is made from polycarbonate and includes a simple locking device at one end that flexes downward for easy release; the new refill is slid through all the jaws and pushed through slight resistance to lock into position. Regardless of the refill type, ensure that all frame jaws are engaged as the refill is pushed into place and locked, as the metal blade holder and frame can scratch the glass if they touch it.