The current heat wave is causing more smog in the eastern portion of the Lower Mainland.
That's led Metro Vancouver to issue an air-quality advisory for this area and the Fraser Valley.
Smog, a.k.a. ground-level ozone, is formed when sunlight reacts with fuel exhaust or solvents in the air. During extended hot periods, this pollution is taken by the winds toward the Fraser Valley.
It's particularly acute in the late afternoon and early evening, which is why the regional district is advising people to avoid strenuous outdoor exercise at these times.
"Exposure is particularly a concern for infants, the elderly, and those who have underlying medical conditions such as lung or heart disease and asthma," Metro Vancouver states on its website. "If you are experiencing symptoms such as chest discomfort, shortness of breath, coughing or wheezing, follow the advice of your healthcare provider. As we are in the summer season with warm temperatures, it is also important to stay cool and hydrated. Indoor spaces with air conditioning may offer relief from both heat and air pollution."
The fire burning north of Harrison Hot Springs is also contributing to a buildup of fine particulates in the Lower Fraser Valley.