Emission factors of formaldehyde and VOCs were determined for two waste treatment plants (WTP) located in the metropolitan area of Barcelona city. Formaldehyde emission factors were determined from the biogas engines exhausts and the process chimneys (after the biofilter process), and VOC emission factors were determined in the process chimneys. Formaldehyde and VOC were dynamically sampled usingDNPH-coated adsorbent tubes with ozone scrubber and multi-sorbent bed tubes (Carbotrap, Carbopack X and Carboxen 569), respectively, using portable pump equipment. Formaldehyde emission factors from biogas engines were found between 0.001–0.04 g s-1. Additionally, formaldehyde and VOC emission factors from process chimneys were found to be between 0.0002–0.003 g s-1 and 0.9 ± 0.3 g s-1, respectively. Employing real emission factors, the expected concentrations derived from the WTPs in their nearby urban areaswere calculated using The Atmospheric Pollution Model (TAPM, CSIRO), and impact maps were generated. On the other hand, ambient air formaldehyde and VOC concentrations were determined in selected locations close to the evaluated waste treatment facilities using both active and passive samplers, and were between 2.5±0.4–5.9±1.0 µgm-3and91±48–242±121 µgm-3, respectively. The concentrations of formaldehyde and VOC derived exclusively from the waste treatment plants were around 2% and 0.3 ± 0.9% of the total formaldehyde and VOC concentrations found in ambient air, respectively.