A boat sails past Keppel container port terminal blanketed with thick smog in Singapore on Thursday. Singapore's air quality reached "very unhealthy" levels on Thursday as thick smog from agricultural fires in Indonesia's neighboring Sumatra island choked the city-state. (AFP/Roslan Rahman)

A thickening, smoky haze cast a shadow over festivities in Singapore on Thursday, as Muslims headed to mosques to celebrate the culmination of the annual hajj pilgrimage and Chinese readied for a traditional harvest festival.

As many Singaporeans stayed at home for the public holiday, the three-hour Pollutant Standards Index (PSI), which measures air pollution in the country, hit 313, the highest level this year. The haze is blowing in from neighboring Indonesia, where forests are being burned to clear the land for farming, causing the annual problem for the region.

Although there are no official air quality descriptors affiliated with the three-hour index, a reading of these levels on a 24-hour index would mean that air quality is in the "very unhealthy" range, according to the National Environment Agency.

High-rise buildings surrounding Masjid Hajjah Fatimah, a mosque in an area historically associated with the island nation's Malay community, were barely visible through the veil of smog.

While some faithful were seen covering their mouths to block out the haze, none wore masks as prayers conducted inside the mosque required them to wash their faces.

Mustafa Muhamad, 61, said the bad air quality caused some of his friends to say prayers to mark the Eid-al-Adha, or festival of sacrifice, at home instead. From a group of 40 usually seen at each of the day's five prayer sessions, the number had dwindled to around 20, he added.

"The haze is very bad, there are less people in the mosque this year. Coming to the mosque to pray used to be very nice because we would mingle around after," the teacher explained.

At the nearby Sultan Mosque, tourists wearing masks were taking pictures of its iconic facade.

Preparations for the Chinese mid-autumn festival on Sunday, when farmers traditionally celebrate their harvest, were in full swing.

One such celebration will feature giant lantern sets, nightly cultural performances and a food street.

"My son is very sensitive to dust and has lung issues, so I'm limiting his outdoor time," said Esther Au Yong, 34, a freelance editor. A trip to the gardens with her 3-year-old son on Friday night, on an outing organized by his kindergarten is on the rocks due to the haze.

On Tuesday, Indonesia's environment and forestry ministry said the licenses of four Indonesian plantation companies were suspended or revoked for clearing land illegally and sparking forest fires.

Some 27 companies are being investigated in connection with the forest fires, Indonesian authorities said, while 140 individuals are being questioned. A Singapore-listed company is among those under investigation.

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