Building more air-tight dwellings is having a deleterious impact on indoor air quality. In a range of recently completed dwellings CO2 concentrations were measured in occupied bedrooms at unacceptable concentrations (occupied mean peak of 2317ppm and a time weighted average of 1834ppm range 480 – 4800ppm). Such high levels confirm that air tight dwellings with only trickle ventilators as the ‘planned’ ventilation strategy do not meet the standards demanded by the Building Regulations. Reducing ventilation rates to improve energy efficiency and lower carbon emissions, without providing a planned and effective ventilation strategy is likely to result in a more toxic and hazardous indoor environment, with concurrent and significant negative long term and insidious impacts on public health. Furthermore, the methodology underpinning the current regulations cannot be considered as creditable. Any researchers operating in this field require to recognise that dwellings have internal doors.