Bangkok has been notorious for traffic jams and pollution as private vehicle usage continued to grow - 40% over the last 10 years. But actual levels of the most dangerous types of pollution - small dust particles that trap themselves in the lungs - have nearly halved, since the government encouraged companies to produce cleaner cars that run on fuel known as gasohol, a mixture of petrol and biofuel made from sugar cane and other plants, and introduced higher taxes to lower the number of motorcycles on the street with two-stroke engines.