The unpleasant odors perceived are mainly due to three gases: carbon dioxide (CO2), sulfur hydroxide (H2S) and ammonia (NH3).
They are the result of the degradation of organic matter in the absence of oxygen, in the tanks and pits of sanitation equipment. It is therefore in the absence of oxygen in the treatment devices that the gases are produced.
Sulfur hydroxide (or hydrogen sulfide) is the most unpleasant, it smells like "rotten eggs". It is also called differently depending on the profession: "Sewer gas" or "Grisou" by the underground miners. In this case, it is produced by the degradation of organic matter accumulated from the degradation of coal.
Ammonia is dangerous for the respiratory tract.

• In case of high temperatures
  In case of low temperatures (below 7°), the formation of Sulfuric Hydroxide (H2S) and Ammonia (NH3) is very low.
  From 15°, the production is important. It should be noted that the temperature in all-water pits or non-collective sanitation systems is between 10 and 22°C depending on the season.
  When opening the lid of a poorly ventilated all-water tank, the H2S concentration can easily exceed 500 ppm ("parts per million").
• In case of high atmospheric pressure and depending on the time of the year
  The bad smells felt are linked to the atmospheric pressure and the time of the year.
  To understand, you must know that when "it is heavy", it is that the atmospheric pressure is high. All the gases without distinction are more often pressed to the ground.
  On the other hand, in rainy and/or windy periods, the atmospheric pressure is low, and the gases are easily blown away by the wind.
  The problem for the user of a sanitation system is that it is in good weather that he wishes to enjoy his garden, and that it is at this time that the discomfort can be the strongest.

• Carbon dioxide (C02)
  It has a slightly irritating smell. It is colorless and heavier than air (Density 1.52). If it has a "greenhouse gas" effect, it also has a useful role since by settling on the ground, it contributes to the growth of plants and vegetation that assimilate it to develop.
• Sulfuric Hydroxide OR Hydrogen Sulfide (H2S)
  It is colorless, flammable and has an easily recognizable "rotten egg" smell. It is also heavier than air (Density 1.19) and accumulates in unventilated low areas. When a poorly ventilated cesspool is opened, its concentration can easily exceed 500 ppm (parts per million). It is produced by anaerobic bacteria that transform sulfur amino acids from our digestion by combining them with hydrogen atoms to form a molecule of H2S. It is very dangerous and sometimes fatal, people with breathing difficulties (asthma) or eye weakness should avoid contact with this gas, even in small doses.
• Ammonia (NH3)
  Ammonia is well known for its ability to scrub sinks and kitchen items. Colorless and irritating, it has a pungent odor in low doses. At higher concentrations, it burns the eyes and lungs. Normally, it is present only in minute quantities in the atmosphere.
  If it is perceived, it means that a source of production is nearby. It is the result of the degradation of nitrogenous compounds present in the residues of our digestion.