The United States has issued warnings about poor air quality in the states of California, Oregon and Washington due to historic major wildfires, says the Washington Post.

According to authorities, the air quality in the state of Oregon is currently among the worst in the world, and in some states even dangerously poor.

In San Francisco, California, people have once again been urged to stay indoors.

It is feared that wildfire firefighting will become even more difficult as the cooler weather ends on Monday and the air becomes hotter and drier.

U.S. President Donald Trump is traveling to California today to explore the devastated area.

Heavy cloud of smoke near Portland. Image: Diego Diaz / ZumaPress / MVPHOTOS