Background: Specific characteristics of particulate matter (PM) responsible for associations with respiratory health observed in epidemiological studies are not well established. High correlations among, and differential measurement errors of, individual components contribute to this uncertainty. Objectives: We investigated which characteristics of PM have the most consistent associations with acute changes in respiratory function in healthy volunteers. Methods: We used a semiexperimental design to accurately assess exposure. We increased exposure contrast and reduced correlations among PM characteristics by exposing volunteers at five different locations: an underground train station, two traffic sites, a farm, and an urban background site. Each of the 31 participants was exposed for 5 hr while exercising intermittently, three to seven times at different locations during March-October 2009. We measured PM , PM , particle number concentrations (PNC), absorbance, elemental/organic carbon, trace metals, secondary inorganic components, endotoxin content, gaseous pollutants, and PM oxidative potential. Lung function [FEV (forced expiratory volume in 1 sec), FVC (forced vital capacity), FEF (forced expiratory flow at 25-75% of vital capacity), and PEF (peak expiratory flow)] and fractional exhaled nitric oxide (FE ) were measured before and at three time points after exposure. Data were analyzed with mixed linear regression. Results: An interquartile increase in PNC (33,000 particles/cm ) was associated with an 11% [95% confidence interval (CI): 5, 17%] and 12% (95% CI: 6, 17%) FENO increase over baseline immediately and at 2 hr postexposure, respectively. A 7% (95% CI: 0.5, 14%) increase persisted until the following morning. These associations were robust and insensitive to adjustment for other pollutants. Similarly consistent associations were seen between FVC and FEV with PNC, NO (nitrogen dioxide), and NO (nitrogen oxides). Conclusions: Changes in PNC, NO , and NO were associated with evidence of acute airway inflammation (i.e., FE ) and impaired lung function. PM mass concentration and PM oxidative potential were not predictive of the observed acute responses.