Many roofs and pavements are dark coloured. In warm and sunny climates these surfaces become hot by absorbing sunlight. This can have the undesirable consequences of increasing the heat flow into the building beneath the hot roofs, which then increases the demand for air conditioning. The hot surfaces also heat the outside air, again increasing air conditioning demand but also increasing the concentration of ozone (smog). We estimate the benefits of replacing (as part of routine maintenance) the present dark surfaces with more reflective surfaces. We conclude that it is a practical way to save energy and to reduce smog in warm climates.