Organisations such as the WHO state that the effects caused by air pollution are reflected on the most vulnerable populations worldwide due to factors such as growth and development that imply a healthier state of people and that the most vulnerable population is children between 4 and 13 years of age because they are in school and recreational environments where the air takes inadequate care of the children's wellbeing. Reviews on the topic of school environments emphasised that indoor air quality (IAQ) is often inadequate in classrooms and causes an increased risk of asthma and other health-related symptoms in the preschool environment. It is more pronounced in developed countries.