The PSI hit a record 321 yesterday, and as the Straits Times puts it, "Singapore endured its worst day of haze yesterday as air quality surged into hazardous territory for the first time". The problem with PSI statistics, however, is that they are based 3 hour averages. As a result, actual hourly air quality could be higher or lower.
For example, the PSI spiked jumped 100 points from 190 to 290 over 8pm to 9pm, so the hourly PSI at 9pm could have exceeded 300, the cutoff for "hazardous levels". Because of this, you might think that the air quality was only "very unhealthy", although it was at "hazardous levels".
I managed to backward derive the hourly PSI from the 3-hourly PSI using the Solver add-on in Excel. The hourly PSI is shown in the chart below. It turns out that the PSI could have hit a high of 450 last night, ouch!