The US Environmental Protection Agency currently ranks indoor air pollution as one of the top five threats to public health. The indoor environment may be as much as ten times more polluted than the outdoor environment and is now considered by many experts to be a major threat to human health, although the air quality will have improved in recent years due to the banning of smoking in enclosed spaces.

Buildings these days are more and more hermetically sealed, as we seek to reduce energy consumption and make the most of the energy we use. The government is encouraging us to draft-proof and insulate old buildings, so the air inside can become a stale and dry, especially in the winter.

When the humidity of indoor air is too low, the air is very dry. This is especially the case in the winter, when the heating is turned up high. Apart from the damage to human health (dry air irritates sensitive membranes in the nose and throat, increasing susceptibility to assaults by airborne chemicals, viruses, mould spores, dust and allergens), this low humidity can cause wood to crack and split.

When the humidity is too high, the indoor environment is damp, and the resulting condensation can cause structural damage. Plants moderate and stabilise humidity and can have a cooling effect in a hot indoor environment. They create a humidity level that is equivalent to the optimum human comfort range of 30-60%.   ......more