… lead emission source. Source: B. Plank / Fotolia.com Lead is a toxic heavy metal and usually occurs bound to other substances. In ambient air lead is measured bound … were used as anti-knock additives in petrol. Car exhausts lead to an immense environmental pollution. Even today road traffic is an important emission source as … were used as anti-knock additives in petrol. Car exhausts lead to an immense environmental pollution. Even today road traffic is an important emission source as …