Asbestos Hood Liner / Bonnet Pad

Asbestos hood liners, also known as bonnet pads in British and Australian terminology, are soft, blanket-like insulation panels fitted to the underside of a vehicle's bonnet or hood. Their dual purpose was to reduce engine noise transmission into the cabin and to protect the hood paint from heat damage caused by the engine and exhaust manifold below. These liners were standard equipment on a wide range of vehicles from the 1940s through the late 1980s.

Unlike the compressed, rigid asbestos products used in heat shields and gaskets, hood liners were typically manufactured from loosely woven or felted asbestos fibre, sometimes blended with cotton or synthetic fibres and backed with a thin layer of vinyl, aluminium foil, or woven fabric. This construction made them inherently friable, meaning the asbestos fibres could be released with minimal disturbance such as touching, flexing, or exposure to engine vibration and heat cycling over time.

The friable nature of asbestos hood liners makes them significantly more hazardous than non-friable automotive asbestos products when disturbed. Over years of service, the combination of engine heat, moisture, oil vapour, and vibration degrades the binder materials, leaving the asbestos fibres increasingly loose and prone to shedding. Opening the hood of an older vehicle and touching a degraded liner can release fibres directly into the breathing zone.

Classic car restorers and hobbyist mechanics are the population most at risk today. Many owners remove old hood liners without awareness of their asbestos content, tearing them off by hand, shaking out debris, or discarding them in household waste. Professional auto body shops and restoration facilities should treat all pre-1990 hood insulation as suspect until laboratory-tested.

Modern hood liners use synthetic acoustic insulation materials such as polyester felt, foam composites, or fibreglass backed with aluminium foil. These provide superior noise and thermal insulation compared to the original asbestos products without any health risk.