Occupant exposure to unhealthy Particulate Matter (PM) in naturally ventilated air-tight dwellings is not fully understood. In the UK, past studies have not investigated PM in bedrooms. Yet, PM is considered the most toxic pollutant and affects more people than any other pollutant; and bedrooms are the spaces that people typically occupy for the longest cumulative periods of their lifetime; with little or no control of ventilation during sleep. This paper evaluates monitored PM10 and PM2.5 in the context of occupant health in bedrooms of eleven dwellings across Scotland. It focuses on PM2.5, the size associated with greatest impact on health. PM and window operation were monitored concurrently. Air-tightness, smoke tests, dwelling inspections, occupant surveys, questionnaires, and interviews were also conducted. The results indicate that PM2.5 concentrations were generally above the recommended limits by WHO; and potentially unsafe in all the dwellings in the context of the EU-ESCAPE study. Bedtime mean concentrations were significantly lower than the 24hr mean, but would also have potentially negative health impacts based on the ESCAPE study. This suggests possible health burdens of particulates in bedrooms, with continuing construction of air-tight dwellings. Further work is needed on a larger sample of dwellings across different seasons.