City design is as yet centered on cars, not people. Although the resulting elevated levels of nitrogen oxides (NOx) cause respiratory diseases and violate EU law, the population appears vastly unaware of the problem. Hence, the web-app visualizes the invisible health threat in real-time, for awareness and empowerment to self-protection. If the NO2-concentration exceeds the legal EU limit, the website’s visualisation turns red. If values stay below the threshold, the screen turns green.