Catastrophic wildfires in France and Spain have forced many to leave their homes. But it's not just the immediate threat to life from the flames or smoke that is putting the health of thousands in jeopardy.

The fires in Spain and France advanced quickly because of tinderbox conditions worsened by successive heatwaves.

Thick black ash coated roof terraces of houses in the north of the centre of Madrid, an AFP journalist saw, with residents forced to keep their windows closed by the smell of…