Abstract: An improved air quality monitoring device provides accurate and reliable predictions regarding likelihood for microbial growth on building walls. The present invention comprises a housing adapted to affix the device to a building wall, a first sensor array placed in close proximity to the wall, a second sensor array and a processing means operatively connected to the first sensor array and the second sensor array. The first sensor array provides inputs such as relative humidity at the surface of the wall, the wall temperature and wall pressure. The second sensor array provides inputs such as relative humidity, temperature pressure and quality of ambient air. Using inputs from the first sensor array and the second sensor array, the processing means generates an output representative of the likelihood of microbial growth and the likelihood that the wall moisture is caused by condensation. A system of networked monitoring devices is also described.