Continuous real-time monitoring of formaldehyde over 5 weeks in two French primary schools: identification of the relevant time resolution and the most appropriate ventilation scenario

An intensive field campaign was conducted to monitor indoor air formaldehyde in two French elementary schools for 5 weeks, where the experimental conditions were modified every week. Formaldehyde concentration was monitored using two portable laboratory prototypes with a detection limit of 1 μg m −3 and a temporal resolution of 2 s. Carbon dioxide concentration, temperature (T), and relative humidity (RH) were also recorded over the whole field campaign. To take advantage of continuous measurement of formaldehyde for post-data analysis, these 2-s raw formaldehyde concentrations were then used to recalculate the average values for several time steps, i.e., 1 min, 10 min, 1 h, and 1 day. This paper highlights that a time resolution of 10 min is sufficient and well adapted to monitor most of the variations of formaldehyde concentrations observed in these two classrooms. The weekly mean formaldehyde concentrations calculated were in the ranges of 25.6–33.8 μg m −3 and 14.7–26.8 μg m −3 for schools 1 (S1) and 2 (S2), respectively, once excluded an unwanted painting event in S1. The results obtained were in excellent agreement with those determined by using the reference method based on DNPH derivatization. The results also revealed that building materials and furniture were two significant formaldehyde emission sources. Unlike most of the studies found in the literature in which formaldehyde is monitored using passive sampling, continuous measurements allowed us to estimate real children’s exposure. When considering only the children’s occupation time, a significant decrease in average formaldehyde concentration ranging between 20 and 34 % was observed for weeks 4 and 5 when the occupants followed specific strategies for natural ventilation. Opening the windows as soon as the CO 2 exceeds the thresholds appears to be the most efficient way to limit exposure to formaldehyde in the two investigated schools. The findings of this paper demonstrate the importance of continuous and accurate formaldehyde measurements with at least 10-min time resolution which could serve as a roadmap for future developments of formaldehyde low-cost sensors.