My understanding is this:

Contrary to popular belief, aircraft cabin air is not "recirculated." The air coming out of the vent nozzles above each seat is air that has been drawn in through the engines, super-heated and then cooled to a comfortable level. In other words, it's fresh.

At the rear of the aircraft (almost back to the tail--behind the rear bulkhead) is a pair of pressure regulator valves. Cabin pressure is determined by a control in the cockpit and, if I remember correctly, is usually set for the equivalent of what the atmosphere would be at 8,000 feet MSL. The pressure regulators release cabin air at a rate to offset the air coming in from the air conditioning system in order to maintain the set cabin air pressure level.

So, since air is coming in throughout the aircraft (at the nozzles above each seat) and is being continuously vented from the rear of the aircraft, logically, the least-polluted air is going to be found at the front of the aircraft. It should be the same on any aircraft, not just the 737 NGs.

But as I said, I'm not an expert...