Annual mean concentrations in urban areas throughout the world are generally in the range of 20–90 μg/m3(15). In the European Community Respiratory Health Survey (ECRHS II) covering 21 European cities, annual ambient nitrogen dioxide concentrations ranged from 4.9 μg/m3 in Reykjavik to 72 μg/m3 in Turin (16). The maximum hourly mean value may be several times higher than the annual mean. For example, a range of 179–688 μg/m3 nitrogen dioxide has been reported inside a car in a road tunnel during the rush hour (15).
Nevertheless, if atmospheric NO2 can be reduced to a minimum it probably should be, but not at any cost and in any event it could take a lifetime to nail down what that minimum should be. This is one of the attractions of environmental science, the problems are interesting and likely to last for an entire career.
Maximum levels associated with the use of gas appliances (gas cooking and heating) in European homes are in the range 180–2500 μg/m3.
So quite high levels. As for the effects:-
Several types of animal study have indicated that nitrogen dioxide increases susceptibility to respiratory infections (60,61,88–90). An extensive set of data was collected using the infectivity model, which measures the total antibacterial defences of the lungs of mice. For long-term exposures, the lowest concentration tested that increased mortality when challenged with Klebsiella pneumoniae was 940 μg/m3 (0.5 ppm) for 3 months of exposure (91). After a 3-hour exposure, the lowest concentration tested that affected resistance to Streptococcus pneumoniae was 3760 μg/m3 (2 ppm) (92).