Canadian wildfire smoke has reached Manhattan, thus resulting in media hysteria regarding what you might think was a dog-bites-man story (filth in New York City).
As measured by METARs at LaGuardia, this was the worst official report:
KLGA 071908Z 31018G22KT 1SM R04/P6000FT HZ FU BKN028 OVC035 19/04
Translation: 7th of month; 3:08 pm; wind from NW at 18 knots gusting 22; 1 mile of visibility (6000’ looking down runway 4); haze and smoke; broken layer of clouds 2,800’ above the airport, overcast 3,500’ above; temperature 19C; dewpoint 4C.
For reference, unless there is fog or exceptionally heavy rain, visibility is usually at least 3 miles.
Note that instrument flying conditions (less than 3 miles of visibility) are often caused by smoke in Alaska and northwest Canada.
From an FAA weather handbook: