Background: A problem associated with the use of pressurised metered dose inhalers (pMDI) is the inappropriate seal between a patient’s lips and the inhaler mouthpiece. Such incorrect use of inhaler may affect its aerosol performance due to unpredicted changes of the flow dynamics in the inhaler. Methods: The current study investigates the in vitro aerosol performance of a Ventolin® suspension pMDI using cascade impaction method, at different experimental setups to simulate conditions where the correct/incorrect use of inhalers occurs. In addition, effects of modified Ventolin® actuators with high and low flow resistances on the pMDI aerosol performances with the above configurations were also evaluated. Results: This study demonstrated that different seal conditions with a normal Ventolin® actuator did not significantly affect the aerosol performance of the formulation (p>0.05). No significant change (p>0.05) in fine particle dose was observed under different combinations of actuators and mouthpiece adaptors. The aerodynamic diameter and distribution of emitted particles from all experimental conditions did not show any significant change (p>0.05), probably due to the similar size of pre-engineered particles in the suspension formulation. Conclusion: It is hypothesized that the additional airflow induced by improper seals between patients’ lips and actuator mouthpiece may not influence the aerodynamic performance of suspension pMDIs. Resistance of air flowing through the actuator influences particle deposition in the actuator mouthpiece and USP induction port, possibly due to different degree of flow turbulence at the exit of the actuator mouthpiece. However the particle deposition profiles in the cascade impactor did not exhibit any significant changes.