The emergency medical services helicopter was about 800 feet above ground level and landing at a rooftop hospital helipad in a congested metropolitan area when the left side engine cowling opened. The cowling partially separated and impacted the bottom of all four main rotor blades, resulting in substantial damage to the rotor blades. The pilot described the event as a brief shudder, like flying through the rotor wash of another helicopter. The pilot was unaware that any damage had occurred and landed uneventfully. A third medical crewmember boarded through the right rear door for a local flight to a suburban hospital. During engine shutdown at their destination, another medical crewmember exited the helicopter on the left side and was the first to notice the damage. The pilot stated that he failed to complete a thorough preflight inspection before the accident flight, because the crew was assigned a medical mission just after their shift started. The previous pilot, who flew the helicopter the evening before the accident, stated that he had opened the left side engine cowling to check the oil level and became distracted. He also stated that he could not remember if he had secured the cowling latches.