A haze has settled over Singapore, causing visibility issues and breathing problems, as the air quality has now reached levels the city has not seen in nearly a decade.

At 10 p.m. local time on Monday (10 a.m. EST, Sunday) the three-hour Pollutant Standards Index (PSI) read 155, the highest level since 1997, when it reached 226, Bloomberg reported. Any PSI between 101 and 200 is considered "unhealthy" by Singapore's National Environment Agency (NEA).

After a bad air quality day on Friday, Singapore residents were hopeful that the air pollution would abate as the weekend saw significantly improved air conditions. But when the city went back to work on Monday, the haze grew worse and worse.

People driving had to use their headlights during the day, and police officers handed out face masks to residents. Some complained of breathing problems.

"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," the NEA said in a statement. "Everyone else should limit prolonged or heavy outdoor activities."

The haze isn't the result of factories, or vehicles, or any other normal pollutants. The NEA attributes it to forest fires on Sumatra, one of the islands that make up Indonesia. The NEA added that drier-than-normal weather conditions have led to what they are calling "an escalation in hotspot activities."

The hotspots are peat and forest fires located at various places throughout Sumatra. However, companies looking to build oil palm plantations typically use these fires to their advantage by partaking in illegal deforestation, according to Mongabay. Many of these companies are owned by firms in Malaysia and Singapore.

The Meteorological Service of Singapore reported 138 hotspots throughout Sumatra — the highest number in at least the past month — and the head of the Environmental Impact Management Agency (EIMA) in the Rokan Hilir regency of Sumatra said there were 33 hotspots recorded only in that regency.

The EIMA plans on telling its residents about the hazards of clearing land by burning.

But until then, winds that are part of the westerly monsoon season will carry the smoke and its smell toward the Singapore region, permeating everything from subways to air-conditioned offices.

The haze can be dangerous for residents. A study found that land-clearing fires and their pollutants in the atmosphere pushed air quality levels in Malaysia and Indonesia 300 percent past World Health Organization (WHO) limits for almost 200 days of 1997. The researchers estimated that these levels could result in about 15,000 deaths — not including infants and children. Between 1997 and 2006, they found that between 5.4 million and 60 million people were exposed to particulate matter (PM) and ozone levels above WHO recommendations.

Although WHO found that there is no safe level of PM under which no damage to health occurs, The WHO Air Quality Guidelines estimates that by reducing PM levels from 70 — the world average — to 20 micrograms per cubic meter, air quality related deaths will be reduced by 15 percent.

Air pollution has short term consequences too. The Environmental Protection Agency (EPA) says that excess ozone — a component of smog — can cause chest pain, coughing, throat irritation, and congestion. It can also reduce lung function, especially in children, because their lungs are still developing and they tend to play outdoors more frequently when ozone levels are high.