In flight school (or if you're both older and diligent about new weather products, when the GFA replaced the FA) you learned that a yellow dashed line on the GFA surrounds an area of reduced visibility in fog or mist. In the north you learn that it might also denote reduced visibility in smoke. While I have encountered reduced visibility in smoke in an urban area due to a mattress factory fire, this usually means forest fire smoke. It's actually a natural state for great swathes of the boreal forest to be on fire at any one time, but because the trees are so valuable in Canada, and because forest fires can threaten towns and cities, we have a vast industry dedicated to detecting, tracking and extinguishing fires. That all takes time, so even with so many resources committed to putting them out there are still a lot of fires. For example here's a map showing the fires burning in Ontario today. The further north you go, the smaller and less valuable the trees get, and the less politically important the settlements are, so the more likely the decision makers are to let the fire just burn. I guess it costs less to evacuate a few hundred people than to fight the fire. So there can be a lot of smoke.
Fog and smoke are very different. Fog sticks around usually for no more than a few hours and while it may render the airports it blankets completely unusable, fog is usually only a few hundred feet thick, so the airspace above it is perfectly usable. Smoke doesn't stop you taking off. Usually visibilities are quite good on the ground and decrease as you climb. Smoke can produce IFR visibilities up to the flight levels and can persist for weeks.
It's sort of cool to see the yellow dashed lines swirling across the map from one GFA to another. In this case its a small burn area, producing lots of smoke and just the wrong winds bringing it to where we wanted to work. We find somewhere else to go, and I check NOTAMs. The runway is reported "90% bare and dry" and it's May. The ten percent is presumably old snow piled up at the sides. Better than April where I got completely snowed in for two days. When it finally stopped snowing I had to get the airport snowplough operator to dig a path so I could get the airplane out. They don't give you dashed yellow lines for low visibility in snow. Precipitation gets green lines. Freezing precipitation gets red lines. Moisture or particles parked in the air and refusing to precipitate get yellow lines. Clouds get brown curlicues. GFAs are very colourful.