Singapore advises citizens to stay indoors as air pollution rises to unhealthy levels on Monday, due to illegal forest clearing in Indonesia. Singapore urges Indonesia to take action.

"Given the current hazy conditions, it is advised that children, the elderly and those with heart or lung diseases reduce prolonged or heavy outdoor activities," Singapore's National Environment Agency said in a statement.

The city-state's Pollutant Standards Index soared to 152 on Monday, Yahoo! News reports. According to the National Environment Agency (NEA) website, the threshold of "unhealthy" is 100 on the index.

It was Singapore's worst haze level since 2006 when the PSI reached 150, statistics from the Ministry of the Environment and Water Resources showed.

According to The Christian Science Monitor, the taste of smoke and the smell of burnt wood was prevalent even in air-conditioned offices and the subway. The usually-clear city was shrouded in thick clouds of gray.

The NEA said it "urged the Indonesian authorities to look into urgent measures to mitigate the transboundary haze occurrence."

Yahoo! News reports that parts of neighboring Malaysia were also suffering from the smoky haze, but the government had failed to take any action thus far.

The haze has not yet affected business or air transport, and Singapore schools are on holiday, according to MSN News.

"The hazy conditions are expected to persist for the next few days," the NEA said.

The Indonesian forestry ministry responded and said firefighters were already fighting the blazes, but water-dropping aircraft would only be deployed if local governors made a request. So far, no such requests have been made.

Singaporean doctor Ong Kian Chung, a respiratory specialist at the Mount Elizabeth Medical Centre, said he expected an increase in patients if the haze remains at current levels or worsens.

"The usual complaints during haze are throat irritation, eye irritation, cough and difficulty breathing," he said.

Those who have pre-existing respiratory conditions, such as asthma or chronic bronchitis, are more at risk, he said.