Many of us don`t have the luxury of working next to an open window - especially as some companies prefer to keep the windows firmly sealed and pump the place with air conditioning! But if you do have the choice, work in a well-ventilated area with as much natural light as possible.
If your options are limited, though, install a plug-in ioniser to help improve the air quality. Ionisers are available from department stores and larger chemists. Also, ensure you get out of the office for at least 20 minutes every day, regardless of your office set-up.