. Author manuscript; available in PMC: 2017 Aug 1.

Published in final edited form as: J Allergy Clin Immunol. 2016 Apr 6;138(2):386–396. doi: 10.1016/j.jaci.2016.02.023

Table 4.

Sources of UFP and their background concentrations in cities and upwind of roadways

Natural sources³⁴
Biological agents (viruses, microbes and fungal parts), combustions, geological processes (volcanic eruptions), and atmospheric transformations (gas to nuclei mode and condensate aerosols)
Anthropogenic sources³⁴
High temperature processes (welding, smelting), combustion (power generation, mobile sources, residential and commercial heating, cooking), and industrial processes
Background concentrations in cities and upwind of roadways
Range	1 × 10³ – 5 × 10⁴ p/cc ^a, ^{33, 34, 36, 43–48}
Peak concentration ^b	8 × 10⁴ – 3.5 × 10⁵ p/cc
Factors affecting UFP levels	Season ^44,47, relative humidity ^43,49, traffic volume ³⁶, ^49,51, vehicle type ^44,46,50, and traffic flow pattern ^44,51

particles/cm³;

within 20 meters of the roadway