You should look for:
A programmable timer - allows you to set an on/off schedule to run before you enter your home or office.
A dirt sensor - to monitor the ratio of air pollution and thus adjust itself.
CADR speed - the clean-air-delivery rate (CADR) tracks the filtration pace. Over 350 is best, below 100 is insufficient.
A servicing indicator - to alert you whether the filters need a replacement or wash.
An ionizer - to amass and trap particles via static electricity.
A fan - makes the difference between a quiet air purifier with a washable filter and a loud one. However, units without fans have poor performance.
Handles and wheels - so you can easily relocate the unit from one room to another.