Wildfire smoke from as far away as Russia and Alaska has caused a thick haze to settle over parts of Metro Vancouver and the Fraser Valley, but health officials in the region say it's what you can't see that could do the most harm.

Ground-level ozone, which is commonly referred to as smog, could cause breathing problems and lung irritation, regional authorities are warning.

"Exposure is particularly a concern for infants, the elderly and those who have underlying medical conditions such as lung or heart disease and asthma," officials said in an air quality advisory that was first issued Saturday and renewed the following day.

Ground-level ozone is not emitted directly into the air, but forms when pollutants emitted by burning fuel and organic compounds from solvents react in the presence of sunlight.

The highest levels of ozone are usually seen between mid-afternoon and early evening on hot summer days.

"If you can stay indoors in air-conditioned areas, that's going to improve your air quality and remove yourself from the heat," said Trevor Smith, a meteorologist with Environment Canada. "But if you are outside, avoid strenuous activities. I guess if you're young, you can go for a 10-mile run. Most people probably avoid that."

Those who experience shortness of breath, wheezing, coughing or chest discomfort should see a doctor as soon as possible.

A heat wave that has gripped much of the South Coast is partly to blame for the reduced air quality. A stubborn bog fire in Richmond is not helping the situation either.

For the past week, temperatures have peaked in the high 20s in communities near the coast and above the 30 C mark further inland.

Metro Vancouver, the Fraser Valley, Howe Sound, the Sunshine Coast, Whistler and parts of Vancouver Island are currently under a heat warning from Environment Canada.

Officials say the smog is unlikely to lift until the heat wave ends. That means the air quality advisory will likely last until at least Wednesday, when daily highs are expected to drop to the low to mid-20s.

Wildfire smoke prompts special air quality statements

The wildfire smoke moving through the province has led to separate special air quality statements from Environment Canada covering every region in B.C. with the exception of Haida Gwaii and a handful of other areas near the Yukon and Alaska borders.

"Individuals may experience symptoms such as increased coughing, throat irritation, headaches or shortness of breath," the agency warned, adding that those with breathing difficulties should stay inside as much as they can.

"Children, seniors, and those with cardiovascular or lung disease, such as asthma, are especially at risk."

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With files from CTV Vancouver's Nafeesa Karim