Pending court approval, several companies affiliated with National Car Rental will pay a fine of $475,000 for repeated violations of motor vehicle idling regulations at two New England airports: Logan International in Boston, Mass. and Bradley International near Hartford, Conn. What is so wrong about idling? Diesel combustion releases fine particles and gases into the air. Commonly called soot, these particles are typically smaller than 2.5 micrometers or 1/30 the width of a strand of hair and are easily inhaled and may cause respiratory harm. By law in many states (especially urban ones) idling too long is an issue.