Older brake pads or shoes may contain asbestos, which has been determined to be a cancer causing agent. Never clean the brake surfaces with compressed air! Avoid inhaling any dust from any brake surface! When cleaning the brake surfaces, use a commercially available brake cleaning fluid.
Fig. Fig. 1: Size 2B photo -- installed view of drum brake components
Fig. Fig. 2: Size 2B photo -- exploded view of drum brake components