Indoor concentrations of PM10, PM2.5, CO, and CO2 were measured in 25 naturally ventilated urban residences during the winter and summer seasons in Alexandria, Egypt. Ambient air samples were also collected simultaneously for comparison to indoor measurements. Furthermore, data for air exchange rates, home characteristics, and indoor activities during sampling were collected. It was found that the average indoor PM10, PM2.5, CO, and CO2 concentrations for all homes in winter were 119.4 ± 30.9 μg/m3, 85.2 ± 25.8 μg/m3, 1.6 ± 0.8 ppm, and 692.4 ± 144.6 ppm, respectively. During summer, the average indoor levels were 98.8 ± 21.8 μg/m3, 67.8 ± 14.9 μg/m3, 0.5 ± 0.5 ppm, and 558.2 ± 66.2 ppm, respectively. The results indicate that the indoor daily averages of PM10 and PM2.5 concentrations were higher than the World Health Organization (WHO) guidelines for all selected homes in the two sampling periods. For CO and CO2 levels, the indoor daily averages for all monitored homes were less than the WHO guideline and the American National Standards Institute/American Society of Heating, Refrigerating and Air-Conditioning Engineers, Inc. (ANSI/ASHRAE) Standard 62.1, respectively. A strong seasonal variability was observed, with air quality being particularly poor in winter. Due to increased ventilation rates in summer, indoor levels of air pollutants were strongly dependent on ambient levels, while in winter the indoor concentrations were more strongly affected by indoor sources due to increased human activities and poor ventilation. In addition, stronger indoor/outdoor correlation of air pollutants' levels was found in summer than in winter probably due to higher ventilation and infiltration in the summer. The study also attempted to understand the potential sources and the various determinants that influence indoor PM, CO, and CO2 concentrations in the two seasons. The findings can assist policymakers to better understand the indoor air pollution problem and to provide a sound basis for the development of proper national IAQ standards in Egypt.Implications: Personal exposure is considerably influenced by indoor air pollution which increases health risks. Assessment of indoor air quality has become a more significant issue in Egypt as people tend to spend most of their time inside buildings, especially in their homes. Currently, there is a lack of research on residential indoor air quality in Egyptian cities in terms of the spatial and temporal variation which prevents an accurate assessment of the current situation to develop effective mitigation measures and to establish national indoor air quality standards. This article is considered the first research studying the effect of seasonality on indoor concentrations of PM10, PM2.5, CO, and CO2 in urban residences in Alexandria. It also studies the indoor/outdoor relationship of air pollutants' levels and identifies their major sources as well as the various determinants that influence their indoor concentrations.