When atoms, which are generally understood to be the smallest unit of matter, cluster together in groups of two or more, they become molecules. A molecule can be changed by energy from things such as the sun or the movement of water. This energy can essentially push a negatively charged electron particle out of the molecule, leaving it with more positive protons than electrons. The extra proton turns it into an ion with a positive charge. An electron that was pushed out may attach itself to another molecule, giving it a larger number of electrons than protons, and turning it into a negative ion.
The difference between an ion and a molecule is the type of charge. Atoms and molecules are neutral, having a balance of positive protons and negative electrons. When that balance shifts, molecules become either positive or negative, depending on whether there are more protons or electrons after the change. Ions do not form on their own, but are created by the effect of some type of energy upon a molecule.
One major source of the energy that creates ions is the sun's radioactivity. Electricity, friction, an open flame or another source of heat all create the type of energy that can produce ions. Negative ions are generally produced in large numbers in situations where water evaporates, so the air after a rainstorm will typically be rich in these ions. The air before a storm, however, will often contain many more positive ions than normal, thanks to the dropping pressure and its effect on the oxygen molecules.
Negative ion therapy is a popular alternative treatment today because some evidence suggests that an atmosphere filled with ions with a negative charge is better for health and well-being. Positive ions are produced by such things as electronic devices and those that use radio frequency, including cell phones and television sets. Naturally occurring winds can pull electrons from molecules, creating positive ions. The same phenomenon can occur in air pushed through a heating or cooling system in a home.
Studies through the years suggest that positive ionization of the air can increase feelings of anxiety and have other negative effects. On the other hand, negative ionization of the air is believed to have the opposite effect. Indoor spaces, in city areas where there's little vegetation and a lot of pollution from traffic — and anywhere heating or air condition is running — generally have low or non-existent levels of negatively charged ions. These places typically have high concentrations of positive ions instead.
Nature produces negatively charged ions in high amounts, largely from evaporating water and sunlight. The faster the water is moving and the more of it that hits the air, the higher the concentration of negative ions will usually be. Amounts of negatively charged ions near waterfalls, for instance, is extremely high, and areas near bodies of water such as rivers and oceans is also rich with them.
Negatively charged ions may help people feel more at ease and promote good health, which explains the popularity of things such as negative ion generators and other gadgets. Likewise, the sense of well-being many people feel after a rainstorm is believed by some to be caused by the high concentration of negatively charged ions in the air as the water beings to evaporate. Additionally, a negatively charged ion inhaled with other oxygen molecules may help oxygen to be more easily absorbed into the body and, in turn, create that sense of wellness, though many experts disagree on whether there is any true benefit.
Even among those who believe that negative ionization of the air can be healthy, there is general agreement that not everyone will benefit. Only some people seem to be sensitive to the beneficial effects. It is also important for someone who purchases an ion generator to make sure that the device does not produce ozone in high amounts, especially if he or she suffers from asthma or other breathing problems.