Particles in the air may be of natural or industrial origin. In high concs, particles can be a serious form of air pollution. Airborne particles are also sometimes referred to as "particulates" or "aerosols". When used in connection with air pollution measurements, these terms all mean essentially the same thing. There are many sources of particles. Combustion sources which result in the release of particles to the air include refuse burning, forest fires, industrial fuel use in boilers, kilns, pulp mills, refineries and power generating stations, and motor vehicles. Natural or non-industrial sources include wind-blown soil dust, sea salt, dust from volcanoes, and fine sand from roads (especially when pulverised by traffic). Other kinds of particles actually form in the air when gases such as sulphur dioxide (SO2) and nitrogen oxides (NOx) react with each other. This is known as secondary, or gas-to-particle conversion. The tiny concentrated droplets that result are partly responsible for the yellowish "smog" sometimes seen over city areas. The pollution sources which generate particles directly often emit the gases which lead to secondary particle conversion, as well.

Particles in the air can be measured in a variety of ways; several standard methods exist. Air quality standards have been developed for some of these measurement methods

The smallest particles can enter the lungs and slow down oxygen uptake, which may in turn put stress on the heart. People with existing breathing complaints such as asthma, bronchitis or emphysema, are most susceptible to adverse effects from high concentrations of particulates. Particles can also cause corrosion and soiling of metalwork and other materials, damage vegetation, and reduce visibility. The impact of particles on visibility is receiving increasing attention worldwide. The enjoyment of scenic vistas may be impossible if haze blots out the view. As a result, some jurisdictions have introduced special programs, some of which are backed up by legislated standards, for visibility protection. Such programs exist in the United States and Australia.

Total suspended particulate (TSP) has been a standard pollution measurement for many years. This method makes no distinction between natural particles, such as pollen and spores, and particles that are emitted from vehicles or smokestacks, nor does it distinguish between the different sizes of particulates which may be present. The maximum acceptable 24 hour TSP value is 120 micrograms per cubic metre, together with an annual standard of 70 micrograms per cubic metre.

Average daily TSP concentrations at sites monitored in Saint John were well within established standards in 1994, with peak values seldom exceeding 45 micrograms per cubic metre. Mean and peak readings were slightly higher at the central Provincial Building site compared to those obtained at the Forest Hills locations. This is most likely due to the greater concentration of dust-generating activities closer to the city centre, and a greater percentage of ground covered with grass and vegetation around the Forest Hills site. There is no clear seasonal cycle at either monitoring location, although there may be a greater probability of high readings in spring or early summer when the roads are often covered with sand spread during the winter, which may be raised by the wind after having been pulverised by traffic.

In 1995, the pattern observed in 1994 was repeated, although with some higher 24-hour peak concentrations seen at the Provincial Building site in summer. The Provincial Building again had higher TSP values in the majority of months.

In Saint John, TSP levels have fallen substantially over the past 20 years. In the late 1970's, annual average concentrations often closely approached the 70 ug/m3 standard, whereas since the mid-1980's, annual averages have seldom exceeded even 50% of this value. The downward trend appears to have stabilized at the Provincial Building since the late 1980s, but there are signs that levels may still be falling slightly at Forest Hills.

The results of health studies over the past decade have dramatically increased the focus on particulate air pollution, and in particular the importance of the smaller sizes of particles. The smallest particles are most damaging to human health because they enter the lungs, whereas larger sizes are screened out by the nose, throat and upper windpipe. Particles which are small enough to enter the lungs are termed inhalable particles. This kind of particulate matter is composed of particles 10 microns in diameter or less, referred to as PM10. A micron is a millionth of a metre. PM10 particles are invisible to the naked eye. They are larger than smoke particles but smaller than dust raised from roads, for example.

PM10 may be measured in several different ways, including the use of specially modified high-volume samplers, and low-volume samplers which draw air through small filters.

Recently published health studies have concluded that PM10 is one of the most important air pollutants in terms of human health effects. Lung function, cardio-respiratory mortality, respiratory illness-related hospital admissions, and a variety of other health measures are related to ambient concentrations of PM10. This association holds true in studies carried out in cities all over the world, yet it is still not clear how fine particles cause these effects. Expert committees in Canada and the United States have over the past several years been putting great effort into the analysis of health data, in an attempt to improve understanding of how particles affect health and for the development of improved air quality standards. This work is continuing, and recommendations on standards (or in Canada, national objectives) are expected within the next year in both countries.

In Canada, only Greater Vancouver has so far adopted standards for PM10. Vancouver adopted the California standards, which are more stringent than the national US standards. Details of particle standards in various countries and other jurisdictions are given in the following table. It is significant that PM10 is the only major air pollutant for which standards are set based not on the chemical composition, but physical characteristics of the pollutant.

Some of the health study results indicate the possible significance of particles smaller than PM10. Particles of 2.5 microns and smaller have been measured for some time, but the available database is much smaller than that for TSP or PM10. PM2.5 is also the particle size of most importance as far as visibility reduction is concerned. Increased attention to PM2.5 data is expected in the future.

PM10 particles in urban regions are typically made up of dust raised from roads and due to tire wear, with much smaller proportional contributions from sea salt, diesel exhaust, and secondary particles formed from NOx and SO2 emissions. Most of the elements found in PM10 are those most common in rocks on the earth's surface, therefore known as crustal elements. In contrast, PM2.5 is usually composed of particles generated by combustion sources, including vehicle exhaust, industrial process and boiler emissions, together with secondary particles and very minor contributions from road or quarry dust and sea salt. The chemical composition of PM2.5 is usually much higher in sulphates, nitrates, carbon and heavy metals compare to the crustal-type chemistry of PM10, which is high in calcium, silicon and iron.

Recent technology has allowed continuous measurements of particle concentrations using either beta attenuation or TEOM monitors. These instruments provide far greater detail on how particle concentrations vary over time, and it is likely that they will be increasingly used in monitoring networks in the future to support the setting of health standards for PM10 and PM2.5.

In 1994, PM10 data were obtained at three sites. As with the TSP data, concentrations were highest at the provincial building, with a few peak values approaching the California/Vancouver 24-hour standard. However, there were no exceedances of these standards at this or any other site. Values at Forest Hills and the Western District Tourism site were considerably lower than those at the Provincial Building and are expected to be broadly representative of concentrations in the suburban environment around Saint John. Higher values may be expected in some restricted locations under the influence of quarrying, rock crushing, road construction and other blasting or grinding operations.

There is no clear seasonal cycle in PM10 concs, but at the two suburban locations, there is some indication that readings are highest in winter and summer, and lower in spring and fall. This may have to do with the probability of dry road conditions occurring together with strong winds, but the trend is not strong enough to warrant detailed speculation.

In 1995, data were available from four locations. The Hillcrest location replaced the Provincial Building site when??. Values were once again low at Forest Hills and the Western District Tourism Office and highest in at the Provincial Building, where there was one 24-hourly.exceedance of the Vancouver standard (in July) and another occasion when it was equalled (in February). Peaks at the new Hillcrest location were intermediate between Forest Hills and the Provincial Building.

It is important to remember that high-volume samplers used to collect PM10 data are only operated once every six days on a nationally-agreed timetable. Therefore each monthly mean is the result of five, or at the most, six measurements. Consequently, there may have been other exceedances which were not measured. The six day measurement frequency is not such a concern over the long term, but is prone to inaccuracies over short periods such as an individual month. New technology such as TEOM monitors will solve this problem as continuous data are obtained with no gaps in the record.

There is some suggestion of a decrease in PM10 annual average concentrations at sites monitored in Saint John since 1990, although the record is relatively short and some site changes have occurred, which complicates interpretation. A slight decrease would be consistent with the trend seen for TSP. The annual values since 1990 have all been considerably below the US and Vancouver standards .