This web-based resource contains recommendations and tools to help communities and design professionals integrate good indoor air quality practices into the design, construction, renovation, and operation and maintenance of K-12 school facilities.
http://www.epa.gov/iaq/schools/index.html

In the US, as far as EPA (Environmental Protection) standards, we speak of good air quality standards, rather than hygienic.

Here hygienic fresh air means clean air which has been filtered and is from germs/dust present in air .This is reheated before it is circulated in the establishment as per the requirement.