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[Display omitted] • High number concentration and toxicity of ultrafine particles cannot be ignored. • Monitoring the number concentrations of small particles is important. • Mono-culture exposure to PM may underestimate the true PM toxicity. Epidemiological and toxicological evidence show that PM 1 is a key driver of PM 2.5. However, the particle sizes that play the major damaging role in PM 1 is unclear. Our aims were to (1) explore the concentration and chemical composition of ultrafine particles in PM 1 ; (2) evaluate and compare the differences in toxicity of PM 1-0.2 and PM 0.2 to different airway cell models. We collected PM 1-0.2 and PM 0.2 in Guangzhou in December 2017, monitored and detected their concentrations, size, and components. A549, THP-1 mono-cultures and A549/THP-1 co-cultured cells were exposed to PM 1-0.2 and PM 0.2 , and evaluated for toxicological responses. PM 0.2 contributed to15% PM 1 mass and 87.67% to particle number (PN). Most of PM 0.2 was PM 0.1 (exceed 75%). Structure of particles in PM 0.2 were mostly spherical, whereas in PM 1-0.2 they were more diverse. Within the same mass unit, PM 0.2 contained more chemical components than PM 1-0.2 ; within the same volume, a higher mass of components was found in PM 1-0.2. IL-6 and IL-8 were secreted by co-cultured cells in higher manner compared to mono cell cultures. PM 0.2 -induced higher toxicological responses compared to PM 1-0.2. Our findings suggest that high PN concentration and toxicity of ultrafine or PM 1 cannot be ignored. Mono-cell culture may underestimate the true PM toxicity. [ABSTRACT FROM AUTHOR]