Ambient concentrations of these pollutants have dropped in some urban areas, but there is still cause for concern.

There are, however, a number of processes and factors in addition to emissions that can influence ambient concentrations.

Public health risks must be assessed in terms of anticipated or measured ambient concentrations.

The ambient concentrations of ground-level ozone have not changed significantly.

NO2 alone, though applied in a sixfold ambient concentration, did not significantly increase plant growth.

Ambient concentrations of particles are typically measured over a 24 hour sampling period, as described above.

Its ambient concentration is usually much less than that which leads to biological toxicity in terms of human health.

Ambient concentrations averaged over the growing season were generally calculated from monthly averages.

Epidemiological studies use field data such as the ambient concentrations of pollutants and their actual effects on a given population.

Ozone causes adverse health effects at ambient concentrations currently observed in Canada.

Such differences typically occur when biological activity acts to change the ambient concentration of oxygen.

It was also felt that relative measures would result in different ambient concentrations at different locations and times.

Expenditure costs rise very steeply as the ambient concentration approaches zero.

Several countries, including Canada, set more stringent standards for pollution emissions in an attempt to reduce ambient concentrations.

Effects occur at indoor ambient concentrations if there are unventilated sources of combustion.

Information on ambient concentrations of lead is to be updated on a three-monthly basis.

Both the ambient concentration and the associated uncertainty within the data set are to be considered in determining the Reference Level.

Ambient concentrations in Montreal were sufficient to cause significant increases in illness and death.

Despite decreasing ambient concentrations, public health risks related to NO2 appear to be increasing.

Both local and regional emissions underlie local ambient concentrations in many urban areas.

Air quality, measured in terms of ambient concentrations of carbon monoxide, is generally good.

Dead mycelia usually did not accumulate atrazine in excess of the ambient concentration.

The map also indicates that some watersheds in the Prairie provinces have elevated ambient concentrations.

Aluminum plant workers are the most exposed, since ambient concentrations in potlines can sometimes reach 1500 µg/m3.

The risk of CH4 gas explosions is highest at ambient concentrations of between 5% and 15%.