Pollutants such as sulfur dioxide and benzene are now largely irrelevant and are measured only in specific cases. Ozone is also not a major problem in the city, as it typically forms outside urban areas during hot weather.

Since 2015, nitrogen dioxide (NO₂) levels in the city have fallen steadily, particularly in high-traffic areas. This is shown in the figure below, which illustrates the trend in the annual average NO₂ levels at the three official monitoring stations of the Hessian State Office for Nature Conservation, Environment, and Geology (HLNUG) in Wiesbaden. There was a particularly sharp decline in 2019. Since 2015, levels at the Schiersteiner Straße and Ringkirche monitoring stations have been reduced by half. In addition to the HLNUG’s measurements, the data series from the Environmental Agency—collected via a mobile monitoring vehicle and NO₂ passive samplers—are shown below. A passive sampler for measuring nitrogen dioxide is a small tube with a special coating on the inside. Without electricity or built-in pumps, this tube absorbs NO2 from the ambient air. The Environmental Agency replaces the tubes every month; they are then analyzed in a laboratory. The results are used to assess air quality.

The positive trend is not solely attributable to the COVID-19 pandemic. Even the closure of the Salzbachtal Bridge did not lead to a renewed deterioration in air quality, despite the additional traffic volume.

Particulate matter pollution has also declined over the past few years and appears to be stabilizing at a low level. For 2024, the annual average readings from the HLNUG’s three monitoring stations for the PM 2.5 particulate matter fraction currently stand at approximately 8 µg/m³, well below the limit value of 25 µg/m³; this is shown in the figure below illustrating the trend in PM 2.5 particulate matter levels from 2015 to 2024. Generally, at these low concentrations, not only pure emissions but also factors such as specific weather conditions play a role and can influence the measured values. Due to high levels, an environmental zone was introduced in 2013.

To improve air quality and protect public health, the European Union plans to establish new EU air quality standards that include stricter limit and target values for, among others, NO₂ and the PM 2.5 particulate matter fraction. The plan is to reduce the current annual average limit value for NO₂ from 40 µg/m³ to 20 µg/m³ and the annual average limit value for PM 2.5 from 25 µg/m³ to 10 µg/m³. These are to take effect starting in 2030. 

Measures such as the increased use of low-emission vehicles (Euro 6/Euro 6d), the expansion of bike lanes, faster bus connections via dedicated bus lanes, and the conversion of the bus fleet to lower-emission models have contributed to improvements and offer further potential for reduction. A digital traffic control system (DIGI-V) is also expected to help reduce pollutants in the future.