Carcinogenicity

Soots are known to be human carcinogens based on sufficient evidence of carcinogenicity from studies in humans.

Properties

Soots are black particulate matter formed as by-products of combustion or pyrolysis of organic (carbon-containing) materials, such as coal, wood, fuel oil, waste oil, paper, plastics, and household refuse. Their chemical compositions and properties are highly variable and depend on the type of starting material and the combustion conditions. Soots vary with respect to their relative amounts of carbon; their particle types, sizes, and shapes; and the types of organic and inorganic compounds adsorbed to the particles. In general, soots have a total carbon content exceeding 60% and a high content of inorganic material and/or soluble organic fraction. The soluble organic fraction of soot is extractable with organic solvents and consists of PAHs and their derivatives. Inorganic constituents may include oxides, salts, metals, sulfur and nitrogen compounds, water, and other adsorbed liquids and gases (IARC 1985; Watson and Valberg 2001).

Soots are classified into four morphologically distinct forms: (1) aciniform carbon, (2) carbonaceous xerogel particles, (3) carbon cenospheres, and (4) coke and char fragments. Aciniform carbon consists of aggregates of rounded particles fused together in random configurations, said to resemble grape clusters. Although particulate emissions from fireplaces consist largely of aciniform carbon, this form is uncommon in chimney soot. Xerogel particles form when organic materials deposited on aciniform carbon are heated, causing the particulate aggregates to be cemented together; this form is common in chimney soot. Carbon cenospheres are hard, shiny, porous or hollow spheres formed when carbonaceous liquid droplets undergo carbonization with little change in shape; they are particularly associated with combustion of heavy fuel-oil sprays. Coke and char fragments consist of carbonized wood or coal and range in size from microns to millimeters; they are common in chimney soots from wood- or coal-burning fireplaces (IARC 1985).

Use

As unwanted by-products, soots have limited uses. They have been used in horticulture and in recovery of trace metals in the metallurgical industry. Weathered soot has been used as a fertilizer to provide small amounts of nitrogen and essential trace metals to plants. Soot has also been used horticulturally as a slug deterrent and as a soil conditioner to increase heat absorption by darkening the soil (IARC 1985).

Production

Soots are not produced commercially. They are by-products of the incomplete combustion or pyrolysis of organic materials (IARC 1985).

Exposure

Humans may be exposed to soots by inhalation, ingestion, or dermal contact. The general population potentially is exposed to soots from fireplaces, furnaces, engine exhaust, and particulate emissions from any combustion source. Occupational exposure to soot may occur among chimney sweeps, heating-unit service personnel, brick masons and helpers, building demolition personnel, insulators, firefighters, metallurgical workers, horticulturists, and anyone who works where organic materials are burned. Chimney sweeps likely have the highest occupational exposure to soots. In 1982, there were an estimated 5,000 chimney sweeps in the United States. In addition to cleaning residential house chimneys, chimney sweeps clean industrial chimneys and boilers and are exposed to both gaseous and particulate combustion products. Soot concentrations to which they are exposed vary depending on the sweeping duties, type of chimney, and type of fuel. In a Danish study, soot concentrations from personal samplers ranged from 4.1 to 388 μg/L for total soot and 1.1 to 25 μg/L for respirable soot. The highest concentrations were measured in chimney ducts where coal was burned and in chimneys where wood and other fuels were burned (IARC 1985).

Regulations

No regulations or guidelines relevant to reduction of exposure specifically to soots were identified.

References

1. Evanoff BA, Gustavsson P, Hogstedt C. 1993. Mortality and incidence of cancer in a cohort of Swedish chimney sweeps: An extended follow up study. Br J Ind Med. 50(5):450–459. 10.1136/oem.50.5.450 [PMC free article: PMC1012164] [PubMed: 8507598] [CrossRef]
2. IARC. 1985. Soots. In Polynuclear Aromatic Compounds, Part 4, Bitumens, Coal-Tars and Derived Products, Shale-oils and Soots. Lyon, France: International Agency for Research on Cancer. pp. 219–241. [PubMed: 2991123]
3. IARC. 1987. Soots. In Overall Evaluations of Carcinogenicity. Lyon, France: International Agency for Research on Cancer. pp. 343–344.
4. Watson AY, Valberg PA. 2001. Carbon black and soot: Two different substances. AIHAJ. 62(2):218–228. 10.1080/15298660108984625 [PubMed: 11331994] [CrossRef]