Ozone is a reactive, unstable form of oxygen, often found at relatively high concentrations during hot, hazy summer weather. Such haze may build up over a period of days into a "photochemical smog". The yellow or brownish colour of such smog is due to fine chemical particles, rather than ozone. Ozone itself is invisible.

Unlike most other air pollutants, ozone is not directly emitted, but forms in the air when strong sunlight acts on a mixture of hydrocarbons and nitrogen oxides (ozone precursors). High concentrations of ozone can irritate the eyes and throat, cause headaches, bronchial irritation and shortness of breath, and produce an acrid taste and smell. Ozone also damages plants, weakens rubber and attacks metals and painted surfaces.

Ozone is responsible each year for agricultural crop losses in sensitive species such as potatoes, tobacco and tomatoes, and can cause noticeable leaf damage in these and other species. Trees and other vegetation may be injured, or growth inhibited, without any visible injury.

Ozone pollution is greatly influenced by weather conditions. A combination of light winds and strong sunshine, persisting with little change over several days, allows ozone precursor compounds to build up with little dilution. Under these conditions, ozone concentrations often rise progressively day by day as long as the warm, stable weather lasts. Anticyclones responsible for this kind of weather can move slowly across the Maritimes in summer and bring ozone pollution from the northeastern United States and sometimes southern Ontario and Quebec. Because of the influence of weather, ozone pollution is highly variable from one year to the next. In warm summers, high ozone concentrations many be experienced repeatedly, whereas during more unsettled seasons, virtually no high values may be recorded.

Ozone episodes mainly affect southern New Brunswick. Monitoring data and meteorological analysis indicate that much of this ozone originates from populated regions of the north-eastern United States. Ozone pollution, unlike most other air pollutants, is worst not in and around large urban centres, but in rural areas. This is because the nitric oxide emitted from motor vehicles reacts rapidly with ozone in the urban environment. Ozone is converted into other compounds during this process. Accordingly, the monitoring network for ozone has sites in rural and suburban areas across southern New Brunswick.

The New Brunswick Department of Environment participates, with Environment Canada, in a Smog Advisory Program, which advises the public of potentially unhealthy ozone levels in southern New Brunswick. Information releases are issued by the media along with routine weather forecasts from May to October. New Brunswick began to offer this service in 1993.

There are major control programs in place in Canada, the United States and other countries which are designed to prevent the occurrence of unhealthy ozone concs. Despite large expenditures on emission control technologies, vehicle emission testing programs and a variety of other initiatives and legislation, these measures have had only modest success, and ozone continues to be a problem pollutant in many areas. It is thought that the growing numbers of vehicles and vehicle miles travelled have offset technological improvements in the emission characteristics of motor vehicles. Emission testing programs have also not been as successful as initially thought. In addition, the chemistry of ozone in the atmosphere is highly complex and is difficult to simulate successfully with computer models, which are the main tools used to devise control strategies. This poses a continuing challenge to air quality managers in their quest for the most efficient means to target pollution sources which contribute to ozone.

At present, many regions in Canada experience ozone concentrations in excess of the 1-hour and 24-hour objectives (82 and 25 ppb). Regions especially prone to exceedances of these objectives include the lower Fraser valley region of British Columbia, including the Vancouver region, the southern Prairie provinces, southern Ontario and Quebec (principally along the St Lawrence river) and southern New Brunswick and Nova Scotia. A national scientific assessment of ozone pollution in all these regions has been underway since 1990. Detailed scientific reports on monitoring, health and vegetation effects, data analysis, ozone precursor emissions and chemical modelling are expected to be issued during 1996.

In 1994, there were significant ozone episodes in July and August, and to a lesser extent in September and October. There is often a seasonal peak in ozone in the late spring, which can be seen in the records from Blissville, Forest Hills and Fundy National Park. This spring peak is due to natural changes in the background ozone concentration in the air. Monthly means at the Customs building site were low, due to the nitric oxide removal effect explained above. Ozone concentrations exceeded the 1-hour standard of 82 ppb at the following sites during 1994: Customs Building, Blissville, Point Lepreau, Norton and Fundy National Park. Several of these sites experienced multiple exceedances. At Point Lepreau, exceedances of the one hour standards occurred in July, August and September. Values of greater than 100 ppb occurred at Blissville, Norton and Point Lepreau in July. In August, a new site was established at Canterbury in a rural area of central southwestern New Brunswick. Ozone concentrations at this site appear to be lower than those observed closer to the Fundy coast.

In 1995, ozone concentrations were generally lower than those in 1994. Peak hourly values in excess of standards still occurred at Blissville, Norton, Point Lepreau, Fundy Park and Canterbury, but the prevalence of peaks over 100 ppb was reduced. The highest value recorded during the season was 104 ppb at Blissville during June.