Factors that may explain greater UFP's proatherogenic potential. UFP (in red) are the smallest, most numerous particles and with best pulmonary retention (top panel). UFP exhibit greater relative content of redox-active compounds (e.g. PAHs, in green) than bigger particles (mid panel). UFP's greater surface-to-mass ratio may allow reactive compounds (in green) to have increased localization towards the surface of the particle (in red) and be more bioavailable for free-radical reactions when in contact with cells (bottom panel). Estimates for the increased content of prooxidant PAHs and surface-to-mass ratio in comparison to PM2.5are from the SCPC study .