PSI table is grouped by index values and descriptors, explaining the effects of the levels, according to Singapore's National Environment Agency (NEA). Note: This chart reflects the guidelines used in Singapore and may differ from other countries. Health advisories are based on the US EPA's guidelines. Only the 24-hour PSI value and not the 3-hour PSI value is correlated to the health effects outlined in NEA's advisories. Record values of the PSI Singapore has been regularly hit by smoke haze from forest fires in nearby Sumatra, Indonesia, brought over by wind. These forest fires have been attributed to the slash-and-burn method favoured by several large plantation owners to clear their land, as opposed to a more expensive and inconvenient mechanical approach using excavators and bulldozers. In June 2013, severe haze hit Singapore, pushing the nation's PSI into Hazardous levels for the first time in its history. Presently, the highest 3-hour PSI reading on record in Singapore is 471 on 20 October 2015 at 11 pm (GMT+8). Association with health outcomes Singapore's computation of PSI and NEA's definitions of PSI ranges have been shown to correlate with a number of health outcomes including all-cause mortality. For sudden cardiac deaths, every increment of 30 units in PSI correlated with 8.15% increased risk of out-of-hospital cardiac arrest on the same day of exposure. This risk was found to remain elevated for 1–5 days after exposure. Similar short-term associations were subsequently found for acute myocardial infarction and acute ischemic stroke in analyses of national registries. In terms of healthcare utilization, both country-wide Emergency Department visits and hospital admissions were increased per unit increase in PSI. References External links Past and present PSI readings in Singapore published by the NEA Air pollution Environment of Singapore Environment of Malaysia United States Environmental Protection Agency Air quality index