Update (12am 28th Sept 2015): We have revised our calculations to what we think is the right implementation of NEA's formula to derive hourly PSI from hourly pm2.5 readings. This is now no longer an estimate!
Update (11.39am 24th Sept 2015): Thanks Mothership
for the feature!
Update (11.14am 8th Oct 2014): Thank you HQ for pointing out a previous mistake in my formula. Calculation has been updated.