1. Lately has been the off/rainy season for smog in Thailand, so levels have been comparatively low... Wait till Dec., Jan. and onward arrive.
It also depends on whether the up to 50 value you're mentioning is actually an AQI reading that you call it, or is instead a 50 reading of micrograms per cubic meter of PM2.5 pollution, which is probably a more common reading used by more air purifiers, since AQI standards can and do vary some by country.
But either way, your lungs will be begging for your air purifier by the end of the year and into next year if you're still in central Bangkok then.
Here's the various AQI readings for Bangkok as of Sunday night -- almost all of them well above the 50 AQI level:
http://aqicn.org/station/bangkok
And, here's what Bangkok's pollution levels by AQI have looked like over the past several years by month, with the red levels being unhealthy, the orange levels being unhealthy for sensitive groups, the yellow moderate, and the green good.