Staying alert at the wheel is obviously critical not only to your survival, but to that of others as well.

A steady supply of fresh air can play a critical role in helping to keep a driver from nodding off.

Two factors come into play: the recirculate position on the HVAC (heating, ventilation, air conditioning) system and air conditioning.

One of the biggest advances in air quality in recent years has also created one of the most dangerous situations -- the recirculate feature. This HVAC control or position, typically identified by a pictograph of the windshield or front of the vehicle with a curved arrow superimposed on it, has two purposes:

To allow occupants to close off the supply of outside air when in the presence of dirty air (i.e. a construction site, or when in the vicinity of commercial vehicles);

And to allow quicker heating or cooling of inside air.

The critical issue is the closing off of the outside air supply. By its very nature, recirculation involves breathing the same air over and over again.

Modern vehicles are very tight and while a certain amount of outside air is mixed in during the recirculation, the vast majority of the air you are breathing is the same stuff you previously exhaled.

With each breath you are removing oxygen from and adding moisture to the air in the vehicle. The reduced supply of fresh air contributes to the likelihood of nodding off or falling asleep.

This is especially dangerous in warmer weather when the sun falling directly on the occupants, combined with the lack of fresh air, creates an ideal environment for a snooze. Even with the fan on full blast and the temperature set at meat locker levels, the diminished supply of fresh air can be dangerous.

The other related issue involving recirculation is the increased moisture it creates puts an extra strain on the system and results in fogged-up windows.

Have you ever noticed how some vehicles, during that time of the year when the outside air is cooler, seem to have all the windows steamed up, some with water actually running down the side or rear windows? We'll guarantee you the air flow lever or button is in the recirculate position and has been for some time.

With every breath, the occupants increase the humidity in the vehicle and when that moisture-laden air comes in contact with the cold glass, you get condensation. If you chose to use the recirculate feature remember to return it to fresh air after the first few minutes of operation. Some of the more advanced system do this automatically, but make it a point to check yours.

Another issue that crops up in relation to fresh air, especially when fuel prices are high, is the debate over the merits of air conditioning in relation to fuel mileage. Everyone "knows" air conditioning increases fuel consumption. That certainly was the case, but is it still?

As with many motoring issues, established wisdom has to be altered with the advances in design and engineering. Aerodynamics now play such a critical role in the design and operation of a vehicle that opening a window to let fresh air in can actually result in higher fuel consumption than running the air conditioning. The added drag causes the engine to work harder and use more fuel than it takes to turn the A/C compressor.

Vehicles are designed in the wind tunnel and a great deal of effort is spent studying and controlling the air flow over, around and under them. It takes power, and thus fuel, to punch a hole through the air and the more slippery designers can make a vehicle, the less disturbance it will create at speed. Open a window and you disturb this carefully crafted airflow.

There are caveats of course. Aerodynamics will not play a major role in the case of someone driving an older car with a roof rack loaded with boxy items and/or a big trailer tied astern.

And aerodynamics only come into play with speed. At slower speeds or when stopped, there is no flow over the vehicle and thus the operation of air conditioning will use a small amount of fuel. Since there is no air movement, opening the window doesn't supply much relief either.

Generally speaking, for most modern cars, minivans, sport-utility vehicles and light trucks, aerodynamics are a critical component and opening a window at highway speeds will be more costly in terms of fuel use than running the air conditioning system.

The advantages of air conditioning are not limited to hot summer conditions. Air conditioning removes moisture from the air making it especially useful year-round in times and climates of extremely moist air.

The cost of operation and possible savings over driving a vehicle with windows open will obviously vary according to vehicle design, speed, and the condition of the system. You won't save enough money to pay the added cost of A/C, but don't forget to take into consideration the higher resale value and the degree of added comfort during the period of ownership.

By using air conditioning and avoiding the use of the recirculate feature, you will not only have a supply of fresh air but create a drier and quieter environment in which to operate the vehicle, reducing fatigue and helping you to stay alert for longer periods at the wheel.

Halifax-based Richard Russell

runs a driving school