BRASILIA, Brazil (AP) — Thick smoke has enveloped extensive areas of the Brazilian Amazon on Thursday as the region grapples with a surge in wildfires and a historic drought.

In Manaus, a city of 2 million, air quality ranked among the worst globally, leading to the suspension of college classes and the cancellation of various activities, including an international marathon.

Over the past six weeks, Manaus and other cities of Amazonas state have intermittently been blanketed by thick smoke, making it difficult to breathe.

The city’s air quality index fluctuated between unhealthy and hazardous levels during the last two days, resembling the conditions in some major Asian metropolitan areas.

Another contributing factor is El Niño, a warming of the surface waters in the equatorial Pacific Ocean, which is expected to peak in December.

Many of the Amazon’s major rivers are currently at historically low levels, disrupting navigation and isolating hundreds of riverine communities.

Saved 60% of original text.