Seasonal variations in release of IL-6 (lane A) and TNF-α (lane B) from alveolar rat macrophages after exposure to the coarse fractions of ambient PM collected in Oslo, Rome, Lodz and Amsterdam (top to bottom). Values are shown as fold increase compared to control levels (IL-6ctr.: 455 – 477 pg/ml, SEM: 172 – 230; TNF-αctr.: 382 – 679 pg/ml, SEM: 13 – 23). Values are mean ± SEM (n = 3).