There is information about which fuels smoke and which don't on the Solid Fuels page, about how to burn wood with low smoke under Wood Fuel and solutions to some problems with the Fireplace Doctor.
Smoke is a suspension of solids in a gas, in the context of solid fuel heating it is simply particles of fuel which have not been completely burned, which are carried upwards by the The Chimney Effect and emitted to the atmosphere.
Dark smoke is partially burned particles of fuel, the result of incomplete combustion. It can be dangerous because small particles are absorbed into the lungs, especially the very tiny ones below 10 microns in size (known as 'pm10s'). White smoke is mainly just tiny water droplets, generated when vapour released during combustion condenses in cool air. Generally, dark smoke is clearly visible against a light sky but difficult to see at night or against a dark background, white smoke is visible in darkness when illuminated but will be more difficult to see against a light sky background. One of the standard ways of assessing smoke emission is by using the Ringelmann Smoke Chart.
In the United Kingdom the Clean Air Act gives local authorities the power to designate 'Smoke Control Areas', known as 'smokeless zones' where the emission of dark smoke from chimneys is forbidden, and gives the Secretary of State (of DEFRA) the power to exempt certain fuels and appliances from the need to conform to smoke control.
General information: http://smokecontrol.defra.gov.uk/
Historical background: http://en.wikipedia.org/wiki/Clean_Air_Act_1956
UK Air Quality Archive http://www.airquality.co.uk/archive/index.php
National Atmospheric Emissions Database http://www.naei.org.uk/
UK Smoke Control Areas http://www.uksmokecontrolareas.co.uk/locations.php
UK Smoke Control Areas - Fuels http://www.uksmokecontrolareas.co.uk/fuels.php?country=a
UK Smoke Control Areas - Exempt Fireplaces http://www.uksmokecontrolareas.co.uk/appliances.php?country=a
Clean Air Act 1993 http://www.opsi.gov.uk/ACTS/acts1993/plain/ukpga_19930011_en
Republic of Ireland
Air Pollution Act, 1987 http://www.irishstatutebook.ie/1987/en/act/pub/0006/index.html
Bituminous ‘smoky’ coal ban and information on regulations and ban areas: http://www.environ.ie/en/Environment/Atmosphere/AirQuality/SmokyCoalBan/
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