In urban buildings, indoor pollutants disperse outward through windows and may reenter other rooms, causing inter-unit dispersion. To address public concern regarding cross-infection risk from infectious individuals in self-isolation during window ventilation, this study quantified inter-unit dispersion of indoor respiratory aerosols in two-dimensional ideal street canyons with aspect ratios (AR = H/W = 0.5–3). Indoor-outdoor coupled airflow under single-sided ventilation conditions was simulated using the RNG k-ε turbulence model. Room air change rate (ACH) was determined via the tracer gas decay method. Dispersion of indoor respiratory aerosols in street canyons and multi-storey buildings was simulated using the tracer gas technique and quantified by reentry ratio (R_k). Infection probability (P) was estimated by the Wells-Riley equation. Numerical results shows that the clockwise main vortex in the street canyon dominates pollutant dispersion. The adjacent room of the source room along the main vortex exhibited the highest R_k values. The maximum observed R_k across all ARs was 5.9%. When quanta generation rate q was as low as 13 quanta/h, P values in only three non-source rooms exceeded 5% with the highest value below 7%. When q reached 161 quanta/h, the maximum P values in non-source rooms approximately ranged from 15% to 58% with increasing AR, which could be controlled to less than 10% if the infected or the susceptible wearing N95 mask. Within the parameter space examined, these findings suggest that excessive concern regarding inter-unit transmission by window ventilation may be unwarranted when proper infection control measures are implemented in the source room.