It is well known that the indoor air quality of buildings is extremely important as about 90% of our daily time is spent indoors . It is also well known that indoor air can be 2 to 5 times more polluted than outdoor air, thus leading to compromise the health of the occupants. This is even more important when considering sealed atmospheres, such as spacecraft cabins, where crews are required to breathe indoor air for months or even years.
This post will allow us to understand the main differences between indoor air quality of buildings and indoor air quality of spaceships. In particular, we will discuss which types of contaminants are present in spaceships, on what do they depend on, which are their sources and how are they detected and eliminated. Most of the data and information come from research carried out on the “Mir” and the “ISS”, two of the most important space stations.
2. SOURCES AND THEIR CONTAMINANTS
If spacecraft are considered as airtight spaces, they can similarly be studied as if they were tight buildings or even submarines. All of them are designed to ensure habitability, protect occupants from the outside environment and ensure control of contaminants inside. Most of these contaminants are chemical contaminants, which mainly come from two sources: hardware and crew. 
2.1 Hardware source
The Hardware source consists mainly of two elements: the spaceship’s system equipment and payloads, which is broken down into :
- Payload equipment: can be characterized as continuous sources, with long-term contaminant emissions. Their chemicals can come directly from the materials used to manufacture them, from their functionality or even from their off-gassing
- Payload experiment: can be characterized as discontinuous sources, with short term contaminant emissions
Mainly, system equipment and payload equipment represent the vast majority of sources of chemical contaminants in spaceships compared to experiments. Therefore, it has to be considered that the sources are mostly continuous ones .
Chemical contaminants coming from hardware can be regulated by considering many aspects as for example:
- Proper selection of the material used to construct the inside environment of the spaceship
- Limitation of the amount of contaminant concentration that is used on-board
- Limitation of 1 g/day of water soluble volatile organic compounds such as alcohols, acetone, and glycols 
2.2 Crew source
When it comes to checking and assessing the indoor air quality of buildings, contaminants emitted by human metabolism are usually neglected. However, the same is not true for spacecraft, as the airtightness of space is more influenced by the source of the crew, which emits a diverse number of chemical contaminants such as :
- CO: Carbon monoxide
- CO2: Carbon dioxide
- NH3: Ammonia
- CH4: Methane
- H2O: Hydrogen
- Several short chain carbonyl compounds
As for system and payload equipment this type of source can be characterized as a continuous source of contaminant generation. However, what distinguishes it from the hardware source is that the crew source cannot be controlled and regulated by any means. In fact, there are no measures to limit contaminants produced by the metabolism, and therefore it will always be a constant source. 
2.3 Contaminants and concentration
The contaminants mentioned in the previous chapters represent only a small percentage of those actually present. In fact, in a spaceship the types of contaminants can be several, and in order to identify an order of magnitude of their percentages of concentrations it is necessary to divide them into different categories based on their functional groups   . Each of the categories is then grouped according to the type of source from which it derives.
For each of these contminants there are certain limits of concentrations that have always to be satisfied, and the NASA provides these values on the SMACs which stends for “Spacecraft Maximum Allowable Concentrations”. 
According to a study carried out at the International Space Station, the following concentration values can be observed (see Figure 1). 
It actually “shows the percent contribution of each function group to the total non-methane volatile organic compound (NMVOC) concentrations measured during ISS Program” . It can be observed that the main categories of contaminants that contribute to the majority of the organic volatile compound concentration are those of alcohol and siloxanes, in the order of magnitude of 80%.
3. PERTURBATIONS AND TRENDINGS
The concentration of contaminants can be influenced by several variables that must always be taken into account, such as: temperature, humidity, pressure, and ventilation flow. Nevertheless, there are other types of perturbations that can significantly affect the concentration of contaminants as docked operations, number of crew, and accidental releases. “Docked operations on-board spacecraft can markedly change the contaminant profile. New contaminants not observed prior to docking can be introduced or contaminants observed prior to docking can be enhanced” .
The effect of docking operations can actually be seen on figure 2, where it shows an increase in the concentrations of siloxanes in the Mir atmosphere after docking and hatch opening. 
It can also be seen that after the docking operations the concentrations doesn’t drop right away, meaning that this type of contaminant is difficult to evacuate.
4. HABITABILITY AND CONTROL OF CONTAMNANTS
The main instrument that ensures the survival of the crew in a spacecraft is the ECLSS which stands for “Environmental Control and Life Support Systems”. It is a life support hardware device that provides clean air and water to the crew throughout the entire mission. This instrument must be clearly designed according to the number of crew, the duration of the mission and its complexity. The ECLSS consists mainly of two parts, the Water Recovery System (WRS) and the Oxygen Generation System (OGS). The WRS allows clean water to be produced from crew urine, cabin humidity and Extra Vehicular Activity (EVA) wastes. The OGS, which is the component we are most interested in for this post, mainly produces breathable oxygen, as well as replacing oxygen lost from airlock depressurization and any experiments on the spacecraft.  
In spacecrafts there are several methods to control and eliminate contaminants. One of these is the trace contaminant control systems through different beds. This is a system that allows the main contaminants, such as H2O water vapor and CO2 carbon dioxide, to be removed using thermally regenerable activated carbon beds, which is an excellent sorbent for removal of a broad variety of airborne organic contaminants. Furthermore, these carbon beds can be developed by using a phosphoric acid impregnant in order to remove contaminants as ammonia NH3. Afterwards it will be possible to convey the remaining exhausted air through a catalytic toxin burner. The exhausted acid gases coming from the combustion such as halogens, nitrogen and sulphur will then be removed by absorption using an expandable LiOH bed (see the system on figure 3). 
Contaminant concentration control can also be carried out by other complementary systems. In fact, in order to remove water-soluble polar atmospheric contaminants, the humidity condensed by the air handling system can be used. This system proves to be more effective than the trace contaminant control system, especially for ammonia NH3. 
Another method to reduce concentrations of contaminants, especially non-lethal ones, is to use intermodal ventilation fans (IMV), which allow the contaminant to be diluted over the entire habitable volume of the vessel, avoiding high concentrations in proximity of the sources. 
This post allows us to understand how indoor air quality in spacecraft is guaranteed, in particular it describes which contaminants are present, how they can be categorized and how they are eliminated. It shows that in order to guarantee the indoor air quality, the main thing is to effectively ensure the balance between the generation and removal of contaminants. We have seen how the generation of contaminants is strongly influenced by the stages of the mission, and this allows us to underline once again that the design of a contaminant removal system depends on several factors.
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