For World Environment Day, each year has a different theme and a different host country. In 2019, the theme for World Environment Day was air pollution and it was held in China. There is an air pollution problem in China where they sometimes have to wear masks in order to be able to breathe and not get sick. However, the poor air quality claims a number of lives every year in China.
Before that, in 2018, the theme was beaten with plastic pollution. It was held in India. Plastic has become a problem because people will throw it away and it takes a long time for it to disintegrate. It seems that the theme applies to the host country in some form or fashion.
This year, World Environment Day will be hosted by Colombia with the Partnership of Germany. As of now, the theme for World Environment Day has not been decided however, it will be released soon.