What are VOCs?
VOC is short for "volatile organic compounds" - in German "volatile organic substances". These are compounds with high vapor pressure and / or low boiling point. The methane gas is often left out here, or explicitly demarcated. 
How are VOCs defined?
Different countries define VOCs quite differently, so no exact one Description, which is universally applicable, available. Generally they are divided into:
→ VVOC - "very volatile organic compounds" with a boiling point between 0-50 ° C and 50-100 ° C
→ VOC - "volatile organic compounds" with a boiling point between 50-100 ° C and 240-260 ° C
→ SVOC - "semi-volatile organic compounds" with a boiling point between 240-260 ° C and 380-400 ° C
→ POM - "associated to volatile organic compounds" with a boiling point above 380 ° C 
Where do VOCs come from?
VOCs are natural and synthetic. Natural sources are e.g. Humans, animals, plants and microorganisms. Synthetic sources are e.g. Furniture, as well as construction, and plastics. The most common natural VOCs are the plant terpenes, which are all derived from isoprene. The most common synthetic sources are mainly road construction and in the plastics industry. Here are mostly organic solvents, such as dichloromethane, chloroform, acetone, ethanol or ether used. Humans have the pheromones as a natural source of VOCs. Here comes the statement "can not smell" ago, because the smell of other people can give us an opinion about this person. Plants use pheromones much more often than humans, so they are used to warn other plants against predators, to announce the maturity of the seed carriers or fruiting bodies, to attract bees and other pollinators or to keep out predators and microorganisms.
What are the problems with VOCs?
VOCs can reach the groundwater, which is particularly evident in settlements with a very developed transport network with a high concentration. In addition, VOCs are very problematic when they occur in high concentration indoors. This can be achieved by e.g. contaminated building materials, molds, or even by room fragrance lamps occur. Health effects include headache and fatigue, but also respiratory irritation and hypersensitivity reactions. These symptoms are titled "Building Disease" or "Sick Building Syndrome". This sensitivity to VOCs is usually the case in humans who have a pre-existing disease such as have an allergy to the allergens in essential oils. But it can also, like an allergy, arise without pre-existing diseases or early signs. In the case of essential oils, substances such as eugenol, linalool, geraniol, citronella or menthol are often the trigger of such VOC sensitivities. These substances are also known allergens and can also trigger contact allergies. In addition, VOCs are partially flammable, so sometimes the essential oils, and can thus lead to explosions at a certain indoor air concentration.
How can you protect yourself from VOCs?
To be protected from VOCs you have to pay attention to some things, so you can:
→ Lower the humidity in the room to prevent mold
→ Ventilate frequently to prevent the accumulation of VOCs in the room air
→ Professionally determine and eliminate possible constructional causes
→ Buy a room air ionizer that removes the VOCs from the breath.
The latter possibilities are quite expensive and expensive in the case of structural engineering causes. One should therefore act as preventively as possible, not corrective. Unfortunately, this is sometimes not possible.