Why are the target values of the WHO stricter than the European limits?

When defining the European limit values the baseline was the following question: 'Which limit values can we impose on the EU-27 to achieve the best possible air quality, which provides maximal protection for the population, in a cost-efficient way?' An extensive cost-benefit analysis was made to address this question. Such an analysis compares the economical costs needed to reduce air pollutant emissions (e.g. installation of soot filters) to the benefits resulting from these measures (e.g. decrease in health insurance expenses by decreased prevalence of asthma). Based on this analysis, the costs and benefits are politically balanced, ultimately leading to feasible limit values. Cost-benefit analyses like this are subject of discussion, as the benefits (e.g. health gains) are expressed in monetary terms, which is not always easy, and the costs to decrease emissions are often underestimated, according to industry. Besides, the EU states the costs may not be out of proportion, because extremely high costs could unhinge the social-economic situation, even though the benefits would still be greater than the extreme costs.