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For information about which fuels smoke: Solid Fuels Page. How to burn wood with low smoke: Smoke Advice Leaflet

What smoke is, and how to spot it
Smoke is a suspension of tiny solid particles in a gas. It is important to differentiate between 'white' and 'dark' smoke. White 'smoke' is usually harmless tiny water droplets formed when steam condenses in cool air, it is more common in colder weather, easily visible against a dark background, but difficult to see against a light sky background. Dark smoke is mainly black carbon and often harmful, it is visible against a light sky but difficult to see at night or against a dark background.

How smoke is generated.
When solid fuels become hot, volatile matter - water or tars - inside the fuel element boils, giving rise to a myriad of sudden tiny explosions which shatter the solid material around them, throwing off tiny fast-moving particles which don't get time to burn properly. The smallest of these particles, below about 10 microns (micron = one thousandth of a millimetre), can get into the lungs. The bulk of smoke particles are pure soot - black elemental carbon - and are relatively harmless, the problem is the half-cooked residues which are left stuck to the particles, seen as a fuzzy coating in the photograph below. The residue from wood is almost all hydrocarbons, which if burned hot enough in a carefully designed stove fired with good fuel, can be broken down into harmless substances. Coal and oil smoke is more of a problem as it often contains sulphur and other compounds which simply cannot be destroyed.

Smoke in developing countries
In many countries a combination of poor combustion on indoor hearths leads to a very poisonous type of smoke, plus lack of chimneys and ventilation means it is inhaled directly, may contribute to around 1.6 million deaths each year, mostly of children under five. (see: World Health Organisation)

A domestic kitchen in rural Nepal, 2013
Image: Glyn Hughes/Soliftec

The variability of smoke
Smoke is hugely variable and very difficult to measure. Differentiating between different types or more- or less- harmful smoke is impossible without very advanced facilities. It is therefore extremely difficult to compare one smoke source with another different type. The table below gives 'official' emission figures for various vehicles and types of heating, from which it is perfectly possible to show that, in terms of smoke, 1 stove = 6 cars, or 6 cars = 1 stove.

Smoke is wildly variable and one poorly-used stove or one badly-tuned truck can have a huge impact. Net CO2 emissions are less uncertain, from which it is possible to present the following..

Net CO2Exhaust dustVOCsNO2      CO2 VOCsDust ParticlesNO2
SOLID HEATper kW.hper 5kW.h output
BAT Stove (Cub)15,630711062393,780603573,738
Euro 2022 Stove15,63012418662393,7801,1166203,738
DEFRA Stove15,63046530062393,7801,8002,3253,738
Older Stove15,63077530062393,7801,8003,8753,738
Pellet Stove (Euro 2022)15,6306011162393,7806663003,738
Open fire, wood15,6301,60050062393,7803,0008,0003,738
Open fire, coal362,8804,7005004,6102,177,2803,00023,50027,660
Barbecue, chimenea, firepit15,6303,80050062393,7803,00019,0003,738
ROAD VEHICLESper kmExhaust dustRoad dustat 30km/hr
'Good' petrol car (Euro 6 2020)181,0005211683805,430,0005,04076511,400
'Bad' petrol car (Euro 3 2000)181,0005212003805,430,0006,00078011,400
'Good' diesel car (Euro 6 2020)173,0005232521,8405,190,0007,56082555,200
'Bad' diesel car (Euro 3 2000)173,00050233001,8405,190,0009,0002,19055,200
Electric car47,50003002741,425,00009008,220
'Bad' truck (Euro I 1995)867,0001208425512,77026,010,0007,6506,120383,100
'Good' Truck (Euro V (2012)867,0003841812,77026,010,0005402,610383,100
OIL HEATINGper kW.hper 5kW.h output
Central heating269,0003,4301,614,00020,580
GAS HEATINGper kW.hper 5kW.h output
Older FE Gas fire (30% efficient)204,0000110680,000367
Gas Fire (2000 60%)204,0010110340,002183
Gas CH 85%204,0020110240,002129
Bottled gas heater (Propane) 90%230,0000150255,556167
ELECTRIC HEATINGper kW.hper 5kW.h output
Any type256,0001,3701,250,00012,500

CO2 Emissions: https://www.gov.uk/..greenhouse-gas-reporting-conversion-factors-2019
Non-exhaust road dust: https://uk-air.defra.gov.uk/assets/ ..Non_Exhaust_Emissions_typeset_Final.pdf
Barbecues, etc. There is no data for these appliances - assumed to be c3g PM per kg based on analysis of 'three stone' fireplaces
Heating levels are limit levels in EN Standards
Electric car is based on Nissan Leaf
Dust emission from vehicles: https://en.wikipedia.org/wiki/European_emission_standards

EXAMPLE DATA: One way of trying to identify wood smoke is to look for traces of levoglucosan, a type of caramel formed when plant material is partly burned. However 'LG' is also produced from coal, cooking, bio-diesel and other sources.

This graph shows av. smoke; the blue line is levoglucosan from 'domestic' smoke from wood, but likely also from coal and cooking, the red line is other forms of smoke. (Measured at Kensington, London) https://kclpure.kcl.ac.uk/portal/files/12227807/Atmospheric_Environment.pdf

See also: Ringelmann Smoke Chart

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