This paper discusses the four main invisible products of fossil fuel combustion: carbon dioxide, sulphur dioxide, nitric oxide and unburned hydrocarbons. If carbon exchanges between atmosphere, ocean and biosphere remain at their recent levels, the CO2 concentration in the atmosphere is likely to double later next century, with a possible rise in global mean temperatures of around 2°C resulting in significant changes in regional climates. The consequences are uncertain, but with adequate research, there should be time to develop the understanding necessary to guide future policies. Sulphur dioxide, acid rain and photochemical oxidants are a more pressing problem. Knowledge here is more complete, but still subject to considerable uncertainties that have an important bearing on the extent and rate of response of affected areas to reduction in acid deposition. However, from the year 2000 onwards, with the replacement of existing plant with nuclear power or new sulphur-free coal-burning technology, United Kingdom sulphur emissions will continue to decline at about twice the pace of their previous rise. Nitrogen oxides contribute to rain acidity, may have direct toxic effects on plants and, when accompanied by hydrocarbons, can generate ozone, itself a damaging pollutant. The chemistry of ozone production is, however, complex, such that intuitive control policies could be counter-productive.