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Chapter 6

Selected Volatile Organic Compounds(VOCs)

Characteristics, sources and effects

Certain volatile organic compounds (VOCs) warrant special concern because they are capable of being transported very long distances in the atmosphere and play an important role in the formation of ground-level ozone and fine particles. Almost all VOCs contribute to ground-level ozone and most do not break down in the troposphere under photochemical reactions. VOCs are defined technically as organic compounds having a saturation vapour pressure greater than 10¹ Torr at 25°C and standard atmospheric pressure.

VOCs are emitted into the atmosphere from a variety of sources, including vehicles, fossil fuel combustion, steel-making, petroleum refining, fuel refilling, industrial and residential solvent use, paint application, manufacturing of synthetic materials (e.g. plastics, carpets), food processing, agricultural activities and wood processing and burning. Specialized, non-routine monitoring and analytical techniques are required to measure VOCs because they are usually present in the atmosphere in gaseous form at ultra-trace concentrations.

VOC monitoring

VOC samples are collected by automatically drawing ambient air into evacuated stainless steel canisters over a 24-hour period (midnight to midnight), following the National Air Pollution Surveillance (NAPS) sampling schedule every sixth day). Concentrations for 143 selected VOCs are reported for each sample. The list of 143 selected VOCs and their statistics appear in the separate Appendix document.

For purposes of this report, data from 1993 to 2000 from eight ambient monitoring stations (Ottawa, Point Petre, Stouffville, Egbert, Hamilton, Simcoe, Sarnia and Windsor) are included in this discussion. The monitoring sites described in this report are displayed in Figure 6.1.

Figure 6.1 Location of Ambient VOC Monitoring Sites (2000)

Benzene, toluene, and ortho-xylene (BTX)

Benzene is a volatile aromatic hydrocarbon – a compound where the double-bond carbon atoms occur in a ring-type pattern – that is primarily used in the production of plastics and other chemical products. Large quantities of benzene are obtained from petroleum, either by direct extraction from certain types of crude oils or by chemical treatment of gasoline. Benzene may cause various types of leukemia, lymphoma, and blood diseases, and is classified as a human carcinogen.

Toluene is an aromatic hydrocarbon that is used to make chemicals, explosives, dyes and many other compounds. It is used as a solvent for inks, paints, lacquers, resins, cleaners, glues and adhesives. Toluene is found in gasoline and aviation fuel. Studies reveal that toluene does affect the central nervous system of humans and animals, however currently there is no evidence to classify it as a carcinogen.

Like benzene and toluene, orthoxylene is an aromatic hydrocarbon. It is released directly into the atmosphere by manufacturers of motor vehicles and equipment, manufacturers of metal cans and shipping containers, and oil refining. Sources of ortho-xylene as a result of human activity include oil refining, motor vehicles, wood-burning stoves and fireplaces whereas natural sources include coal tar, oil, forest fires and plant volatiles. Although orthoxylene does affect the central nervous system as a depressant, it has not been classified as a carcinogen due to lack of evidence.

BTX are all colourless liquids which are considered highly toxic in small concentrations and are very reactive in forming ground-level ozone and fine particles. Figure 6.2 shows trends in benzene, toluene, and ortho-xylene for the period 1993 to 2000. All three VOCs show a decreasing trend over the eight-year period. However, concentration levels for benzene and toluene increased slightly between 1999 and 2000, while ortho-xylene remained at 0.6 µg/m³ since 1998.

Figure 6.2: Trend of Benzene, Toluene and Ortho-xylene Concentrations (1993– 2000)

Note: Annual composite mean based on eight ambient sites over eight years.

Butane, ethylene and isopentane

Butane and isopentane are classified as alkanes whereas ethylene is classified as an alkene. The difference between an alkane and an alkene is in their structure: alkanes are saturated hydrocarbons in which all carbon atoms form a single bond with other atoms, whereas alkenes are unsaturated hydrocarbons in which some carbon atoms form a triple bond with other carbon atoms. Butane, ethylene and isopentane are emitted primarily from automobile exhausts and by the petrochemical industry. Unlike BTX, these hydrocarbons are not as reactive in forming ozone except when present at very high concentrations.

Figure 6.3 shows trends in butane, ethylene and isopentane for the period 1993 to 2000. All three VOCs have declined since 1993, however butane and isopentane have increased slightly between 1999 and 2000.

In general, VOC concentrations at ambient sites in Ontario are at levels well below existing ministry criteria levels.

Figure 6.3: Trend of Butane, Ethylene and Isopentane Concentrations(1993– 2000)

Note: Annual composite mean based on eight ambient sites over eight years.