SANTA MARIA, Calif. (AP) — Rising temperatures on Wednesday posed new challenges for firefighters who have made incremental progress against a massive wildfire in central California that has injured four people as it has become the biggest blaze in the state so far this year.

More than 800 structures are being threatened by a massive wildfire burning through Los Padres National Forest in central California

The Gifford Fire, a wildfire burning in Central California that has destroyed over 83,000 acres in six days, continues to rage and is now the largest blaze in the state this year, according to Cal Fire.

Smoke from Canadian wildfires is causing air quality concerns for millions of Americans in the Midwest and Northeast.  Air quality alerts were in effect Tuesday in several states, including Michigan and New York.

A thin veil of smoke from wildfires burning in Canada is worsening air quality and putting a lid on temperatures in the D.C. area. The smoke, which has resulted in some of the world’s worst air quality in the Upper Midwest and Northeast, is only wafting through the Mid-Atlantic in limited quantities, but it’s enough to have a small impact.

Each New England state has seen an uptick in the number of days over the last five years when fine particulate matter (PM2.5) or ozone exceeds air quality standards that are considered “unhealthy for sensitive groups.