Automobiles contribute the largest to the smoke concentration in the atmosphere, discharging several hazardous air pollutants, among them :

Nitrogen Oxides – produced whenever fuel is burned, these emissions contribute to the formation of photochemical smog and acid rain.

Hydrocarbons – contains hydrogen and carbon atoms and are present in the combustion product of fossil fuels, causing photochemical smog. HC emissions are released from motor vehicle exhaust and are usually the result of unburned or partially burned fuel.

Carbon Monoxide -a toxic gas produced when fuel is burned incompletely. Road transport account for more than 90% of CO emissions in Metro Manila.

Carbon Dioxide – a by-product of the combustion of fossil fuels and it contributes to global warming when in large quantities.

Sulfur Dioxide – present in gasoline (or, petrol) and diesel fuels but concentrations are higher in the latter diesels than in gasoline (although legislations in the in the Philippines have reduced the levels to permissible). Sulfur dioxide is an acidic gas and contributes to acid rain and smog.

In addition to air pollutants such as NOx, HC, CO and CO2, which are generally emitted in large quantities, smaller amounts of other potentially toxic substances are present in the atmosphere that can have harmful effects on environment and health. Some of these are:

Lead – this metal has been added to gasoline (petrol) to improve engine performance. When burned, lead is emitted. Lead is a known poison that accumulates in humans and affects the development of children. However, emissions of lead are falling due to reduced lead content in leaded gasoline and the widespread use of unleaded fuel. Diesel fuel has no lead content.

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Beneath murky water

Benzene – is naturally present in crude oil and, like lead, acts as an octane enhancer, which prevents the fuel from igniting prematurely and damaging the engine; Benzene is present in gasoline exhaust, and to a lesser extent, evaporates during refueling or operation of the vehicle. Benzene is a known carcinogenic (a cancer-causing substance).

Aldehydes – a group of chemicals emitted from cars as a result of incomplete combustion; Although all aldehydes are suspected carcinogens, formaldehyde is the most significant.

Particulate Matter – tiny (10 microns or less) airborne particles that can carry toxic substances, including heavy metals and chemicals, and cause irritation to the lungs exhausted by diesel trucks and buses.

Polycyclic Aromatic Hydrocarbons (PAHs) – found in exhaust emissions from internal combustion engines (both gasoline and diesel engines), though more so in diesel engines.

Peroxyacetyl Nitrate (PAN) – a product of the complex reaction between oxygen gas and unburned hydrocarbons, PAN is a toxic and lachmatory (produces tears) substance that make up some of the chemicals in photochemical smog.

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