The first Canadian National Ambient Air Quality Objectives were developed in the mid-1970s. These objectives were set for various air pollutants. The NAAQO had three levels indicating severity (maximum desirable, acceptable and tolerable levels) and also evaluated effect levels (maximum desirable, acceptable and tolerable levels).
In 1992, a review of the NAAQOs suggested that many air pollutants had no effect thresholds (e.g. tropospheric ozone or very low effect thresholds). As such, scientifically defensible effect levels could not be identified. In 2000, the revised NAAQOs framework specifies a Reference Level, above which there are demonstrated effects on human health and/or the environment, and an Air Quality Objective (AQO), which is meant to be protective for the general public and environment and which also considers aspects of technical feasibility. This approach is different from how the National Ambient Air Quality Standards are set in the US, where cost is explicitly not considered in the proposal to strengthen the air quality standards to protect human health and the environment. In addition, the US NAAQS are periodically reviewed, frequently strengthened, commonly enforced and allow serious legal penalties if the standards are not met. In contrast, the Canadian NAAQOs are infrequently reviewed, rarely strengthened, not enforced, and the only penalty for not meeting the Canadian NAAQOs is shame.
The original NAAQOs have not been formally revised to the new two-level system. As an interim approach, data are compared with the existing desirable and acceptable NAAQOs for sulphur dioxide, carbon monoxide, nitrogen dioxide and ozone.
The air quality objectives must be consistent with the Canadian Environmental Protection Act (CEPA) 1999. They must also be based on scientific principles, such as risk assessment and risk management. The NAAQOs are set by the federal government based on recommendations from a National Advisory Committee and Working Group on Air Quality Objectives and Guidelines. Provincial governments have the option of adopting these either as objectives or as enforceable standards according to their legislation.