The particle concentrations are measured in five offices of the High Court building with the optical airborne particle counter Light House 3016 IAQ. The daily particles concentration measurements present two major peaks, one in the beginning of the morning and the other in the first hours of the evening. We focused our attention on the night peaks because it has been verified that they cannot be attributed to any inside human activity. Looking for an explanation for those peaks, we used the outside PM2.5 and PM10 concentrations measurements which are performed by the Guadeloupe Air Quality Network with two TEOMs, respectively at about 5km and 500m off the building. They show peaks in mass concentration at the beginning of the night. Comparing the inside and outside night data we obtained a good concordance between particles counting and mass concentration. So we can attribute the night inside peaks to outside particles injected in the office environments through the Mechanic ventilation. The rapid response of the inside concentration relatively to the outside concentration explains the important concentration measured in the morning as the addition of both inside and outside particles sources: reemitted particles from the cleaning operation early in the morning and the beginning activity of the offices in addition with the outside morning maximum pollution of the town. This transforms the office into a toxic environment.