High carbon dioxide concentration in a small area, such as a passenger car cabin, is a health hazard. For many years the outside air flap door on most HVAC systems was notched, so that in recirculation there was some “fresh” air flowing into the cabin. Even without the notch, the car body was relatively leaky and the blower switch didn’t have an off position, only a low speed to purge stale air.
To improve A/C performance, the recirculation switch (or Max A/C position on HVAC switch) in newer cars permits shutting off outside air. Now there’s even a U.S. EPA fuel economy credit because this approach improves A/C fuel economy. However, just for passenger comfort, in states with high ambient temperatures, shutting off outside air is common.
Comfort level just 0.1%
This has an obvious effect on passenger compartment air quality, and CO2 buildup from human respiration can affect passengers. As Dr. G.D. Mathur, senior manager for test and development at CalsonicKansei North America, pointed out in a 2017 SAE World Congress (WCX17) presentation, just 0.1% concentration is the comfort limit. EPA’s short term exposure limit (15 min) of 3% and a maximum exposure of 4% in the breathing zone was promulgated only for R-744 air conditioning (carbon dioxide used as a refrigerant), to cover a large leak, not a human-caused buildup.
ASHRAE (American Society Heating, Refrigeration and Air Conditioning Engineers) says the comfort limit for CO2 concentration is 700 ppm over the ambient level, which is approximately 400 ppm, for a total of 1100 ppm (0.11%).
CO2 sensors provide one avenue for automotive control, but more likely is the timed approach used by some car manufacturers (10-20 minutes at a time in recirculation). However, there is a need to maintain maximum recirculation to meet the intent of the EPA credits.
For all the modeling and the limited testing that has been done in this area, Mathur noted that better data is needed to cover vehicle ageing. Most new vehicles start life with triple sealing of the doors and glass areas, but seals deteriorate over time. A researcher can model CO2 buildup based on number of passengers against cabin volume, air leakage and blower flow rate. However, there is great variability in exhalation CO2 for passenger activity level (sitting quietly vs. parent screaming at youngster in high activity, for example).
There also is a major difference in human lung capacity, and work on R-744 air conditioning systems has led to studies on that subject, showing a range of 3.8% to 5.8% CO2 (38,000-58,000 ppm) in human respiration. Mathur’s research led him to quantify lung capacity at 1.65 L/min, which he said matched well with previous work he had performed. It indicates a buildup to 1100 ppm –just over the comfort level—within the first 4-5 min of a simulated test drive. With a vehicle range of over 500 mi/800 km, an eight-hour trip can raise CO2 concentration to dangerous levels.
CO2 effect on car crashes
Mathur noted several deaths recorded by the Arizona Dept. of Transportation were blamed on crashes from CO2 buildup affecting the driver. The attributions were validated by blood analysis of the crash victims, indicating the issue is real world.
Although he had no specific data, Mathur said that research also needs to consider possible contributions from carbon monoxide (CO). He observed that in recirculation there is no positive pressure in the cabin, so with exhaust and underbody seams leakage, CO can penetrate. The level would be subject to great variability based on the exhaust system and car. If it reaches a level of 30 ppm, it is likely to cause passenger headaches.
Prior to Mathur’s presentation, the SAE Interior Climate Control Committee had discussed this subject at its last meeting and a call was issued for a working group. The purpose was described as to focus only on occupants respiration, not leakage from an R-744 system. Participants would agree on vehicle interior volume, passenger volume, air exchange rate, drive cycle, also engine off and at idle. Testing would be performed with a CO2 cylinder, and specific settings for vehicle sensors and HVAC operation, including possible preconditioning.