According to the plot of my story, terraforming earth-like planets turned out to be an incredibly difficult task and exact copies of the Earth with an atmosphere of oxygen are rare, so it was decided to genetically adjust the children of the colonists to local conditions, namely for breathing chlorine or hydrogen sulfide.
Typical nucleosynthesis processes in stars create more even elements than odd ones, and more light elements than heavy ones. Chlorine is a relatively heavy element, and odd. On average, it is more than 20,000 times rarer than oxygen, an element normally secreted by photosynthesis in the life-bearing inner system worlds. However, there are small regions of the galaxy in which the interstellar medium is rich in chlorine. This is especially true for second and third generation stars, which have a higher "metallicity" (percentage of elements heavier than hydrogen). In the case of chlorine worlds, the interstellar medium from which the star and its attendant planets were formed should also be especially enriched with the so-called elements of the Alpha process (sulfur, chlorine, argon, calcium, Titanium and chromium). If this relatively rare process of nucleosynthesis was common enough in the stars that formed the starting material, then the chlorine world might be possible.
Even if the starting materials are enriched in chlorine, the planetary process must follow a rather unusual series of events to create a chlorine world. Hydrogen chloride is volatile compared to water, and unlike water, it is not often combined in bulk with silicates (hydrated "wet" silicates are the main source of water in the formation of terrestrial planets). The band of material, cold enough to contain sources of hydrogen chloride, and at the same time warm enough to allow the formation of a rocky planet, is very narrow. Within this narrow strip, the planet retains only water in its hydrosphere (although there may be unusual concentrations of more refractory chloride minerals); outside of this strip, gas giants or ice worlds are the rule. Most often, the chlorine world is formed at the outer boundaries of the life zone of the system or even outside it. As the star ages and heats up, or as the dynamics between interplanetary material and planets bring their orbits closer to their original orbits, the chlorine world becomes possible.
The atmosphere of the mature chlorine world consists mainly of oxygen and nitrogen with a significant proportion of chlorine (about 5%). Chlorine is found primarily in the lower atmosphere because it is heavier than other primary gases and has a higher relative abundance in dry areas because it eventually reacts with water to produce hydrochloric acid again. Only the highly efficient and vigorous photosynthetic activity of plants and bacteria supports the supply of free chlorine. The atmosphere is opaque, but has a yellow-green tint from chlorine and various forms of chlorocarbons; on some chlorine worlds, the dense atmosphere also favors the growth of airborne life forms, which can give the lower atmosphere a cloudy, smoky appearance. Clouds are composed of different proportions of water droplets or hydrogen chloride. The weather is complicated by the fact that hydrochloric acid evaporates more easily than water and freezes much more easily. The rain is always sour, but its concentration varies considerably. Ice and snow are rare, and even very cold bodies of water rarely freeze. When they do this, the acidity rises sharply under the water ice due to the removal of water relative to hydrochloric acid.
Here we return to my question: These genetically modified colonists, breathing chlorine or hydrogen sulfide, can periodically leave their planets to meet other peoples of people, sometimes visiting planets with an atmosphere of oxygen. Therefore, when staying in a different atmosphere, they must be able to switch from one type of gas (oxygen) to another (chlorine or hydrogen sulfide), how to do this?