The precise extent of the illegal trade in fuel became apparent in data issued by the General State Chemical Laboratory (from 2,000 fuel samples taken from customs inspections): – 32.4 percent of diesel samples contaminated. – 4.8 percent of unleaded gasoline samples contaminated. – 4.3 percent of heating fuel contaminated. – 3.6 percent of special category gasoline contaminated. Of the samples of diesel fuel classified as «not normal»: – Most were contaminated with heating fuel. – Others contaminated with shipping fuel. – Significant percentage of unknown origin and use. According to a Petrochemicals Department official, the latter is usually heating or shipping fuel that has undergone a change of color and had the tracer removed. General State Chemical Laboratory officials say that the combustion of these products is the reason for the rise in emissions of sulfur dioxide, airborne particles and hydrocarbons that cause acid rain and serious respiratory problems.