Any suggestions for getting more fresh air into my office?

I rent a small office which I run my business out of. The trouble is that although there is a very large window, only the top part of the window opens. Most days there is very little air flow into the office and it actually makes me or anyone else working there drowsy, presumably from the lack of oxygen?

Are there any plants or devices which I can use to spruce up the atmosphere? or any other suggestions?

Possibly the prevailing wind direction is wrong for your window, but to get a breeze coming in the window, there has to be an exit point. Figure out a way to open a door or something at the opposite end of the building. That should do the trick.

Not convinced? Try this in your car on a breezy day. Open the driver's side window a couple of inches and see how breezy it is (while stationary). Now close the window by an inch and open the passenger window an inch. Same total opening, but the breeze is greater.

For devices, avoid ordinary air conditioners as they are a waste of money. Instead, look out for a heat pump device that will warm your hot water supply while chilling the ambient air. It should work out cheaper than ordinary direct water heating AND cool the air. That works better with the window closed, of course, so the air isn't freshened.

Large, dark green leafed potted plants work pretty good (the cheap ones at supermarkets work great). Also try an air purifer like the ionic breeze, they clean the air, but they also circulate the air.

The building you're in has an HVAC (heating, ventilation, air conditioning) system. They're all adjustable as to the amount of outside air they take in. Frequently adjustable for local areas. People often set that too low, in order to "save money". Turning it up is easy. Your best solution, if you can get management to do it.

Otherwise figure out how to get a fan sucking air through the gap. Maybe build a stand. Air purifiers will do nothing for low oxygen. You'd need to cover the office with plants to get much relief.

Any green plant sat in the sunlight would help.