Children were exposed to biodiverse material or non-diverse material in sand pits. The two arms were compared. Immune response and bacterial markers were followed.

Children of age 3-5 were randomly divided into two groups (arms). One received guidance and biodiverse material for 20 minutes each day for two weeks. The other received the a similar material and guidance but the microbial community in the material was poor in diversity. Bacterial community changes and immune system markers were analyzed and compared between the two arms. Potential changes in Interleukin 10 was the primary outcome.

Inclusion Criteria:

• healthy
• born in Finland
• spends daily (5 d a week) several hours in one of the daycares in which the trial was performed

Exclusion Criteria:

• medication affecting immune system function
• medication affecting microbiota
• cancer
• immune deficiency
• living on a farm
• not born in Finland

The local ethical committee and the law in Finland retricts the sharing of information that could lead to distinguishing a study participant. Microbial information is considered to faciliation the identification of study participants. Microbial information will be partly shared to public databases (sequences).

Participants nor their parents did not know the arm. Care providers did not know the group as they did not know the content of the sand box. Investigators and field assistants knew the content in each sand box but they did not know the coding of the arms when they analyzed the differences in cytokine levels. Outcomes assessors i.e. nurses who took blood samples did not know the arms nor the contents of the sand boxes.