The reason is that Poland aims to make it’s air clean and compliant with Polish and European air quality standards. This aim is achievable. However, the following changes need to be introduced and enforced.
Many old used European cars recently became available in Ukraine. Almost all of them had a long exploitation life and almost all of them don’t match the sanitary standards and have a high level of air pollution. With such cars, it would be now impossible to travel from Ukraine to Poland. And the main reason is that Poland launched a new national programm “SMOG”.
This program, announced by the Polish government last fall, is worth around 35 billion euros. The funds were coming from the EU budget, the national one as well as money Poland gets from the European emissions trading system.