The latter include health-damaging and smog-forming airborne pollutants like hydrocarbons, nitrogen oxide, particulate matter, carbon monoxide and formaldehyde, as well as greenhouse gases that contribute to climate change, like carbon dioxide, nitrous oxide, methane and other compounds.

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They promise lower emissions of nitrogen oxides, sulfur oxides, particulate matter, and carbon dioxide (CO2).

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"As there is no carbon in the fuel, hydrogen-powered vehicles produce no carbon monoxide, carbon dioxide or particulate emissions, " said Wrightbus business development director Jonathan Poynton.

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Using gas cookers, or burning candles, for example, both result in indoor levels of carbon monoxide and particulate matter that are just as high as those to be found outside, amid heavy traffic.

ECONOMIST: Indoor pollution

Unlike particulate matter and smog-forming chemicals, carbon dioxide can't be filtered or trapped.

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Many students of climate change argue that, although the artificial release of greenhouse gases such as carbon dioxide works to raise the global temperature, particulate pollution should, by promoting cloud cover and thus reflecting solar energy back into space, have a compensating cooling effect.

ECONOMIST: Weather and pollution