As fires from nearby Indonesia intensify, Singapore's air quality has approached hazardous levels, sinking the city into a thick gray haze. The pollution - which has reached its worst level this year - prompted officials to close all primary and secondary schools Friday and distribute antipollution masks to the city's most vulnerable.

Over the last month, forest fires and peatland fires in parts of Indonesia's Sumatra and Kalimantan have sent smoke into Singapore, which is an annual problem but last week the fires intensified and winds in Singapore propelled the situation, CNN reported.

"The air quality worsened into the 'very unhealthy' range today. The PSI for the next 24 hours is expected to remain 'very unhealthy,' and may even go into the 'hazardous' range," Singapore Prime Minister Lee Hsien Loong wrote on Facebook on Thursday.

"Meanwhile, we have offered Indonesia our help to put out the fires, and asked them to share with us the identity of companies responsible for causing the haze," the prime minister also wrote.

"This is the first time I'm experiencing the haze firsthand," Gokulnath Durai, a tourist from Chennai, India, told Bloomberg. "Today it really has worsened. You feel that."

Residents have kept their windows and doors shut, Iain Craig, a British middle-school teacher who has lived in Singapore for six years, told CNN.

"When I open the balcony door, you can immediately smell it. I can feel it in my throat," he told CNN. "Exercising outside is simply not an option right now. . . . All my school football team matches have been canceled over the last week. Kids are brought inside at lunchtime too."

Public events are also suffering. "The haze is very bad. There are less people in the mosque this year," Mustafa Muhamad told the Associated Press. Muhamad had gone to the Hajjah Fatimah mosque to pray on Thursday's Muslim festival of Eid al-Adha.

"Coming to the mosque to pray used to be very nice because we would mingle around after," he said.

There are also concerns about the effect the haze will have on tourism and commerce. "If it lasts long enough, domestic retailers and commerce will show visibly reduced earnings," Wai Ho Leong, a Singapore-based economist at Barclays P.L.C., told Bloomberg. "After that, it has to last long enough for people who have booked trips to Singapore to cancel. There's still a sense this is temporary."