As corporate average fuel economy requirements that started in 2012 continue to tighten, automakers have turned to an entire arsenal of tactics to reduce the fuel their vehicles burn.

Downsized and turbocharged engines, transmissions with many more gears, lighter-weight structures, better aerodynamics and more all add incrementally to efficiency.

One of the less-discussed methods is the "Eco" button offered in increasing numbers of new cars, which reduces the car's available power and minimizes accessory use.

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Among the many vehicle systems an Eco setting can affect is the car's heating, ventilating, and air conditioning system, known by the acronym HVAC.

Now, in an article published by the Society of Automotive Engineers, one vehicle engineer has suggested that reduced ventilation inside new vehicles could potentially lead to dangerous buildup of carbon dioxide, slowly imperiling occupants.

One state transportation department has specifically cited excess carbon-dioxide levels in the blood of more than one driver who was killed in vehicle crashes, taking the problem from the theoretical to the documented.

2010 Toyota Prius - EV, Eco, and Power mode buttons

2010 Toyota Prius - EV, Eco, and Power mode buttons

Released in April, the article notes that for many years, even the "recirculate" setting on a car's HVAC system let in a small amount of fresh air via a notch in the outside air-flap door that closed off the system.

In previous decades, car bodies were relatively leaky and fan blowers often didn't completely switch off, meaning that at least some fresh air was continually admitted to the cabin.

With strenuous efforts to reduce aerodynamic drag and energy draw, vehicle doors are now triple-sealed and certain HVAC settings permit the fan to be entirely switched off, effectively eliminating any fresh-air ingress.

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Moreover, automakers can earn a small credit on the CAFE scores for any vehicle that entirely shuts off the fan on its maximum air-conditioning setting, which slightly reduces its energy usage.

A presentation at the 2017 SAE World Congress technical meeting by G.D. Mathur, a test and development engineer at Calsonic Kansei North America, offered more details.

It noted that EPA limits on short-term carbon dioxide exposure were intended only as safety measures when carbon dioxide is used as a refrigerant in vehicle air-conditioning systems—and were not written to consider the effects of CO2 built up from human breathing.

Carbon dioxide (Image: DJ Spiess,

Carbon dioxide (Image: DJ Spiess,

More research into breathing rates, the durability of sealed cabins over a car's life, and the other factors affecting CO2 buildup appears to be required.

Variations in the number of passengers, the size of the passenger cabin, air leakage over time, and the output of the blower fan will lead to great variability in exhaled carbon dioxide.

As the article notes, a passenger sitting quietly exhales less than a parent yelling at a very active and squirming youngster, for example.

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Mathur's modeling suggested that carbon dioxide could build up to 1100 parts per million—slightly over the comfort limit of 0.1 percent of cabin air—in as little as 4 to 5 minutes of a simulated test drive.

An eight-hour road trip without stopping could thus raise carbon-dioxide levels to hazardous concentrations.

Indeed, the Arizona Department of Transportation has attributed several crash deaths to excessive CO2 buildup in drivers.

Nissan Titan IIHS crash test

Nissan Titan IIHS crash test

Post-mortem blood analyses validated the diagnosis, taking cabin buildup of carbon dioxide from a theoretical issue to an actual safety hazard.

The Interior Climate Control Committee of the Society of Automotive Engineers has formed a working group to focus solely on the role of occupant breathing in cabin air levels and CO2 concentrations.

Meanwhile, the old advice applies: if you start to feel drowsy while driving, open a window.

Any slight losses in fuel economy are a small price to pay to ward off the possible effects of hazardous carbon-dioxide buildup.


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