It is mainly dust consisting of quartz (a silicate which is one of the main constituents of sand) , calcium-rich particles, sulfates (salts of sulfuric acid), hematite (a type of iron oxide or rust), soot, carbonaceous materials.
The calima comes from the Sahara desert in North Africa. It is swept up high in to the atmosphere by desert winds. It is estimated that 50% of the global dust emissions originate in the Sahara and the dust is carried to many parts of the world including South America. The desert regions of northern Africa are estimated to release about 800 million tons of dust each year.
Calima particles can measure less than 10 microns (0.01 micrograms), they can get into your lungs and are small enough to enter the bloodstream. A short amount of exposure is probably not a problem but you should avoid prolonged exposure and doing strenuous exercise when there is a lot of dust in the air. Wearing a mask may help.
Yes, slightly. The French did nuclear tests in Reggane in central Algeria in 1960 which were 3 times as powerful as the bomb dropped on Nagasaki in 1945. It is possible to detect caesium 137 which is a man-made radioactive isotope which was created during the tests. The level of radiation deteced in the calima is not considered to be dangerous for human health. Interestingly, there was no caesium 137 prior to 1942 so it has been used to verify the age of wines and sedimentary deposits.
If rain is forecast after the red dust has settled it may be a good idea to let the rain wash some of it away because water is precious in Spain and it is best to use as little as possible. The best way of cleaning the calima is by blasting it with a jet of water from a hose or from a pressure washer. The most important thing to do is to look at the weather forecasts so that you don't spend a lot of time cleaning up only to have to do it again a few days later. The filters on air conditioners may need to be cleaned and it may be a good idea not to use air conditioners if the calima is very bad, fine dust particles can get into the moving parts.
If possible it is best to avoid letting the red substance get into the filter. Add floculant to the pool at half a litre per 50 m3. Switch off the pump. Give the floculant at least 12 hours to do its work. When the red substance has settled use the bottom cleaner to clean up the pool. Change the valve on the pump so that it expels the water directly to waste water without going through the filter.