A searing heat wave, a bog fire in Richmond, and wildfires in Eurasia and Alaska have led to an accumulation of ground-level ozone, i.e. smog, in parts of southwestern B.C.

As a result, Metro Vancouver has issued an air quality advisory for the eastern parts of Metro Vancouver and the Fraser Valley.

"Ground-level ozone is not emitted directly into the air," Metro Vancouver states on its website. "It is formed when nitrogen oxides (pollutants emitted when fuels are burned) and volatile organic compounds (emitted from solvents) react in the air in the presence of sunlight.

"The highest levels of ground-level ozone are generally observed between mid-afternoon and early evening on summer days."

The smog is expected to stick around for a few days.

The B.C. government's Air Quality Health Index forecasts ratings of 4 for later today, tonight, and tomorrow in northeastern, northwestern, and southeastern Metro Vancouver.

Southwestern Metro Vancouver and the central and eastern Fraser Valley are expected to reach 5 later today, tonight, and tomorrow.

The AQHI reports on health risks not only caused by smog, but also particulate matter and nitrogen dioxide.

Ratings from 4 to 6 are characterized as "moderate risk".

This means that people with heart or breathing problems should consider reducing or rescheduling strenuous activities outdoors.

Ratings from 7 to 10 are considered "high risk", and anything above 10 is "very high risk".

According to Metro Vancouver's Caring for the Air 2017 report, air quality has been improving in recent years even as the population has grown.

Sulphur dioxide emissions have fallen sharply as a result of stricter regulations on marine fuels. Carbon monoxidide and nitrogen dioxides are also down over the past deacade whereas smog levels are rising.

"This is partly due to an increase in ozone formed outside of Canada coming into our regions," the regional government stated in the report. "Metro Vancouver and partners have developed a ground-level ozone reduction strategy to address this issue."

Ground-level ozone levels still remain below where they were in the 1980s and early 1990s.