I'd say that 3 ppm is fine, studies reflect it is normal to see some CO2 in a vehicle, see below. I have never had an issue with my vehicle, fact is the complaints about smells etc. is from .03% of Explorers on the road, the other 99.7% no complaints filed. I can see why the police vehicles have issues when you start drilling holes in vehicles and don't seal them you are going to create issues. I have seen the pictures of those holes, really negligent by third party installers, lazy. Police vehicles often sit and idle for long periods, writing out tickets etc.., if you recirculate the air, don't seal the holes well no wonder you have problems.
I have driven back and forth to Florida twice in my vehicle, 12-14 hrs a day of driving , two days in a row and had no issues, no smell. I don't use re-circulate on A/C , perhaps that is why. I am sure there are some with issues, in the bigger picture not sure it is a big problem but for a small group.
This NIH (National Institutes of Health) Study monitoring CO inside the cabin of a vehicle during commutes over an extended period has some pretty interesting data. This vehicle was tested prior and no CO intrusion was found so it was all about being in traffic etc...
https://www.ncbi.nlm.nih.gov/pubmed/7921891
Carbon monoxide exposures inside an automobile traveling on an urban arterial highway.
Ott W1,
Switzer P,
Willits N.
Author information
Abstract
Carbon monoxide (CO) exposures were measured inside a motor vehicle during 88 standardized drives on a major urban arterial highway, El Camino Real (traffic volume of 30,500-45,000 vehicles per day), over a 13-1/2 month period. On each trip (lasting between 31 and 61 minutes), the test vehicle drove the same 5.9-mile segment of roadway in both directions, for a total of 11.8 miles, passing through 20 intersections with traffic lights (10 in each direction) in three California cities (Menlo Park, Palo Alto, and Los Altos). Earlier tests showed that the test vehicle was free of CO intrusion. For the 88 trips, the mean CO concentration was 9.8 ppm, with a standard deviation of 5.8 ppm. Of nine covariates that were examined to explain the variability in the mean CO exposures observed on the 88 trips (ambient CO at two fixed stations, atmospheric stability, seasonal trend function, time of day, average surrounding vehicle count, trip duration, proportion of time stopped at lights, and instrument type), a fairly strong seasonal trend was found. A model consisting of only a single measure of traffic volume and a seasonal trend component had substantial predictive power (R2 = 0.68); by contrast, the ambient CO levels, although partially correlated with average exposures, contributed comparatively little predictive power to the model. The CO exposures experienced while drivers waited at the red lights at an intersection ranged from 6.8 to 14.9 ppm and differed considerably from intersection to intersection.(ABSTRACT TRUNCATED AT 250 WORDS)
