Govt can only advise, no employer will ever tell their employees to not come to work. They can only tell you up to your own discretion.
In 1997, when it hit 226 on PSI, I still remember going to work. That one really choking.
In a way it's a catch 22 thing with this haze. If ever it goes beyond 300, again it's up to you. You take MC, year end appraisal affected. By then, your boss forget the reason for the MC, only how many MC you get in 2013.
Essential services staff still need to report for work. I think something like a nuclear meltdown like Chernobyl in our backyard then everyone need to stay indoors. Where to run?