Structure of the four barns and sampling points.

Each barn consisted of 32 pens and was accessible via two doors. Exhaust air outlets in each barn are given as circles. Slashes on both sides of the picture symbolize the fresh air inlets of barn 3 and 4. Triangles indicate the fresh air distributors of barn 1. Dust was measured at two sampling locations that were randomly selected on the day of sampling but were identical with the sampling points for air sampling. The latter sampling points (n = 6) are indicated by stars. Stars in yellow indicate sampling points for ammonia (NH₃). A (barn 1 and 3) and B (barn 2 and 4) represent the positions of the sensors for CO₂ and relative humidity. Recirculating air filter modules in barn 4 are represented by a framed X.