Inversion weather conditions strongly enrich the urban air with pollutants. Figure 2/20 shows a model of the dependence of pollutant concentrations on the altitude of the inversion boundary based on a 1985 winter emissions situation in Stuttgart. The higher the altitude of this lower inversion boundary is, the greater is the volume of the mixed air above the city. In addition, the concentrations of various pollutants depend upon the altitude of the typical source. Although traffic-produced carbon monoxide (CO) decreases as the lower boundary of an inversion rises, the sulfur dioxide (SO₂) concentration rises, since higher sources now reach the mixed air and long-distance transport of pollutants now becomes perceptible.

Since the middle of the 1960s pollutants in Baden-Württemberg have been analyzed regularly. Although reductions in the levels of sulfur dioxide and particulate matter are recognizable, levels of the pollutant nitrogen dioxide (NO₂) have increased further. This is due primarily to continuously increasing traffic. In cities above all, the threshold values of the TA Luft publication (1986) are in part exceeded in street canyons (BAUMUELLER, REUTER, 1993).

The immission threshold values IW1 (for long-term effects) and IW2 (for short-term effects) outlined in the TA Luft serve for the air-hygienic evaluation of developments that require permitting. As such, establishing that a development meets the TA Luft values is necessary for the development to be approved. In this process, the arithmetic average of the immission values (labeled as I1Value) is compared with the long-term value IW1. In contrast, the short-term value IW2 refers to 98% of the cumulative frequency distribution. The TA Luft values are not a legal planning instrument; they define rather the legal concept of "damaging environmental effect." The TA Luft values IW1 and IW2 are only valid in connection with the respective processes set forth in the TA Luft for reporting on immissions. These refer to individual test surfaces typically 1 kilometer square inside the area affected by the development in question. Admittedly, as the only existing official threshold values, the TA Luft values are often carried over to other evaluation scenarios.

The 22^nd BImSchV’s limit values for nitrogen dioxide and partikulate matter (PM10) have attained particular importance, as these require measurements taken directly on and near roads. Heavily-trafficked city streets frequently exceed these threshold values.

The World Health Organization has developed health-related indicator values (WHO, 2006).

The definition of "maximum immissions values" (MI values) (VDI Regulation 2310) from the VDI commission "Air Quality Maintenance" aims at avoiding damage, whether short- or long-term, to the health of humans, especially children, the elderly, and the sick, and further aims at protecting animals, plants, and goods from harm. The concept of "health" also includes the well being of a human, to which one’s biological and material environment contributes. The MI values are concerned with purely effect-related, scientifically established values guided by practical experience and with a medical or natural science indication. The values do not take into account technical practicability.

For the evaluation of comprehensive air pollution levels it further makes sense to introduce an air pollution index (API), which takes into account multiple air pollutants (BAUMUELLER, REUTER, 1995; MAYER et al., 2002).

Table 2/5: Threshold and guideline values in µg/m³

I1:	arithmetical annual mean value
I2:	98- Percentil during 1 year
a	depending from the PM value
b	depending from location; I2-values as a 14 days mean
c	18 exceedings are allowed in the year
d	only for 2 successive values
e	on successive days
f	10 - min. mean value
g	PM10 depending on location; I2-values as 14 days mean
h	partly transition period, partly alert values
i	allowed 35 excceedings per year
k	180: limit value to inform the population 240: imit value to warn the population
l	allowed 24 excceedings per year
m	allowed 3 excceedings per year
n	fine particulate
o	allowed 25 days excceedings per year