At Forest Hills and Point Lepreau, sampling for volatile organic compounds (VOCs) was carried out during 1994 and 1995. VOCs are carbon-containing substances which are present in the air in gas form, usually at low concentrations (in the parts per billion range). See box for details. The VOC sampling program in the Saint John region is operated in collaboration with Environment Canada.

The New Brunswick VOC sampling sites form part of a national network of over 50, including locations in urban and rural areas. In large centres such as Vancouver and Toronto, VOCs are monitored at multiple sites. Samples are collected in specially cleaned cylinders and analyzed by Environment Canada at their labs in Ottawa. Sampling for VOCs requires sophisticated field and laboratory equipment, and obtaining reliable data is technically challenging. Due to the high cost, the usual sampling frequency is one 24-hour sample every 6 days. More frequent sampling is carried out at times for shorter periods. The Forest Hills site is considered an urban, industrially-impacted site due to its location in east Saint John, whereas the site at Point Lepreau was chosen to obtain data representative of a rural environment which is seldom downwind of the city.

Thousands of different VOCs exist in the air. However, many VOCs are of natural origin, and in terms of environmental protection, only the VOCs which cause some kind of problem are significant. Sometimes this is because they are themselves toxic (e.g. benzene and formaldehyde) or because they can react in the air to form other harmful substances. For example, ethylene, propane, toluene and butane (and many other VOCs) may take part in reactions which lead to the formation of ozone-containing smog. For ozone and smog studies, VOCs are often defined as compounds containing at least one carbon atom and having a vapour pressure of 0.01 kPa at 25C, excluding carbon monoxide and carbon dioxide. Vapour pressure is a measure of how readily something evaporates. Everyday examples of VOCs include gasoline, varsol, nail polish remover (acetone), chloroform, rubbing alcohol, butane, propane, and naphthalene. Some of these compounds are gases at ordinary environmental temperatures, and those that are liquids all evaporate readily, entering the gas form.