On the nature of airborne aldehydes in a middle eastern megacity: Tehran, Iran

•Formaldehyde (FA) and acetaldehyde (AA) measured in autumn 2018 at a major bus terminal in Tehran.•Mean concentration of FA/AA was 109.99 ± 23.47/85.4 ± 17.17 μg m−3.•The FA:AA ratio was 1.31, with both species linked to vehicles and biogenic emissions.•Statistically significant relationship between FA and AA and the number of buses.•Health risk analysis reveals FA and AA levels exceeded an acceptable limit. This work summarizes characteristics of acetaldehyde and formaldehyde in autumn 2018 in the ambient air of a major bus terminal in Tehran, Iran. The average (±standard deviation) concentration of formaldehyde/acetaldehyde was 109.99 ± 23.47/85.4 ± 17.17 μg m−3. Major sources of these species were traffic emissions, photochemical processes, and biogenic sources (e.g., deciduous-coniferous tree). The average inhalation lifetime cancer risk (ILTCR) values based on IRIS for both aldehydes were between 7.07 × 10-5 and 5.09 × 10-4, which surpass suggested EPA magnitudes. The mean ILTCR values based on the office of environmental health hazard assessment for these species ranged from 1.21 × 10-4 to 1.02 × 10-3, which exceed suggested values by EPA for eleven different age groups. Hazard quotient (HQ) values of formaldehyde were between 7.27 ± 1.61 to 1.34 ± 0.29, while those of acetaldehyde were between 6.68 ± 1.49 and 1.58 ± 0.355. Hence, the ILTCRs and HQs of these toxic species were higher than 1 × 10−6 and one, respectively, which is not at "an acceptable risk for humans". The results of the current work have relevance to the health of the general public close to ‘hot-spots’ like the study region where a large number of people are in contact with carcinogenic pollutants.