Lethal heat waves will become a growing problem in a heating world. We could prepare for extreme heatwaves also by storing salt.  Meaning salts that can be used to reduce the humidity of the air in life-threatening situations.

Most salts are strongly hydrophilic (or hygrophilic). In other words, they are moisture-absorbing materials. Even ordinary table salt which is mostly sodium chloride can absorb moisture from the air relatively effectively. It costs almost nothing and can easily be dried in the sun and reused, over and over again. Unfortunately, sodium chloride can only reduce the relative humidity of the air to 74 per cent and absorbs much less than its own weight of moisture. Even this could be lifesaving during extreme heatwaves, especially for families who cannot afford electric air-conditioners and the related electricity bills, or for cities whose electric grids might break down for days during a major heatwave.

Calcium chloride, a substance known in the North as road salt, is a much more effective moisture trap. It can absorb several times its own weight of moisture before all of it has been converted to brine. Above all it can, at least in theory, reduce the relative humidity of the air to 50 per cent, which is much comfortable than 74 per cent during an extreme heat wave.

According to the HUMIDEX index, measuring the combined impact of humidity and heat, lowering the relative humidity of the air from 90 per cent to 50per cent is roughly equivalent to lowering its temperature by 15 or 20 degrees Celsius.

Calcium chloride is almost as cheap as sodium chloride, only costing less than euro 100 per ton. If it gets into the eye, it causes more irritation than table salt, so it is advisable to be somewhat careful when using it. A temperature of 250 degrees Celsius is needed for drying and reusing calcium chloride, but this can be reached by numerous different simple solar energy devices. Another alternative is to use the brine for storing wood or other biomass, to keep its carbon permanently out from the atmosphere.

Calcium chloride cannot reduce the temperature inside the house, but it can lower the humidity so that the temperature becomes more tolerable for humans. Lowering the humidity of the air can also reduce the amount of heating needed during the winter in northern countries or high mountains. The combination of high humidity and cold is almost as lethal as the combined impact of heat and humidity.

Finnish members of InSIC have also experimented, successfully, with dried sphagnum mosses and lichen. Their experiments have proven that both lichen and sphagnum mosses can reduce the relative air humidity inside a house almost as effectively as calcium chloride.