Smoke from wildfires burning across the Pacific Northwest is no longer the only thing in the air that has health officials in Metro Vancouver and the Fraser Valley concerned.

An air quality advisory issued last week over the high concentrations of fine particulate matter in the region was expanded Wednesday to include ground-level ozone, or smog.

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

The highest levels of smog are usually seen between mid-afternoon and early evening on hot summer days and could cause breathing problems and lung irritation, regional authorities warn.

The added alert comes on the eighth day of an air quality advisory triggered by fine particulate matter, or PM2.5 particles, in the air as thick haze from wildfires burning across the province wafts into the Lower Mainland.

PM2.5 refers to solid or liquid particles in the air that are 2.5 micrometres or less in diameter. These particles easily make their way indoors due to their small size.

The poor air quality has local drug stores selling out of medical masks as concerned residents try to protect themselves.

"We are very much having trouble keeping them in stock," said Dave Woogman, a store manager with London Drugs. "The very young and the elderly, people with respiratory problems, they need to get out of the house."

Unfortunately, experts have said during past wildfire seasons that any mask that isn‘t a is unlikely to have any benefit when it comes to keeping the particles out of your lungs.

For those visiting the area, the haze is hindering world-famous views of Stanley Park, the Lions Gate Bridge, the North Shore mountains and other iconic features of the Vancouver area.

"It‘s a bit sad that we can‘t see anything," a German tourist told CTV News.

Experts say children, the elderly and those with pre-existing conditions such as asthma, diabetes, lung disease or heart disease are most at risk of experiencing adverse effects.

Health authorities say the potentially harmful effects of both fine particulate matter and ground-level ozone are best avoided by limiting strenuous outdoor activities.

The advisory is not expected to end until there is a significant change in the weather. But with some rainfall in the forecast for Friday, Saturday and Sunday, relief might finally be in sight.

For the latest forecast and weather warnings, check out CTV Vancouver‘s new (and free) weather app! Click to find out more information including how to download it to your smartphone or tablet.

With files from CTV Vancouver‘s Sarah MacDonald