International Space Station Air Pollutant Study Could Improve Spacecraft Design

The concentrations of potentially harmful chemical compounds circulating in the International Space Station (ISS) exceed those found in the floor dust of an average American home, according to a new analysis of the “space dust” trapped by the station’s air filters.

The new findings, which included the identification of these airborne contaminants and the consideration of possible sources, could have significant implications for the design of future manned spacecraft. The research is published in the journal Environmental Science and Technology Letters.

Inside the ISS

The ISS is a unique environment. The air inside the space station is constantly recirculated, with 8–10 changes per hour. This air is treated to make sure that CO₂ and trace gaseous contaminants are removed before it is recirculated, however, it is unknown how effective the system is at removing airborne chemical contaminants.

As the ISS has been inhabited by humans continuously for more than two decades, particularly close attention has been paid to the flammability of the contents of the ISS. This includes the use of bespoke, industrial flame retardants that would be less common in the living quarters of an average Earth-dweller. But on top of this, the ISS astronauts may also bring cameras, power tools, clothing and other pieces of everyday equipment into the space station.

The high levels of ionizing radiation that the ISS is exposed to has the potential to accelerate the aging of certain materials, breaking down plastic objects into nanoplastics more rapidly than on Earth. In the ISS’s microgravity environment, these tiny particulates – as well as other degradation products from niche flame retardants and other equipment – can easily become airborne pollutants.

The air filtration system within the ISS is equipped with screen-covered high-efficiency particulate absorbing (HEPA) filters that are intended to remove airborne particles before the air is recirculated. These screens accumulate debris during operation and so require weekly vacuuming to ensure that they are clear to operate efficiently. The debris collected during vacuuming is largely made up of clothing lint, hair, airborne particulate and other debris that is referred to as “spacecraft cabin dust”.

To support studies looking at the environment inside the ISS, some of these vacuum bags have been brought back to Earth by astronaut crews returning from their expeditions.

Persistent organic pollutants found in space dust

Given the complexity and uniqueness of life on the ISS, scientists have hypothesized that the relative abundance of known airborne contaminants in the ISS could be markedly different to regular indoor environments on Earth.

To investigate further, this study used gas chromatography-mass spectrometry (GC-MS) and liquid chromatography quadrupole time-of-flight mass spectrometry (LC-QTOF-MS) to analyze samples of the cabin dust that were returned to Earth.

Several prominent, potentially harmful chemical contaminants were identified in the dust samples, including: polybrominated diphenyl ethers (PBDEs), hexabromocyclododecane (HBCDD), “novel” brominated flame retardants (BFRs), organophosphate esters (OPEs), polycyclic aromatic hydrocarbons (PAH), perfluoroalkyl substances (PFAS) and polychlorinated biphenyls (PCBs).

Under the United Nations Environment Programme’s Stockholm Convention, PCBs, some PFAS, HBCDD and the penta- octa-, and deca-BDE commercial formulations of PBDEs are officially recognized as being persistent organic pollutants (POPs) and so are heavily regulated on Earth. Some OPEs are also under consideration for restriction by the European Chemicals Agency.

Commercial products and industrial flame retardants are likely sources of contamination

While the study authors write that their current analysis “cannot yield direct insight into the origins of dust contamination,” they do offer some speculations as to the likely sources of contamination.

BFRs and OPEs are used in many countries to meet fire safety regulations in electronics and electrical equipment, as well as insulation, fabrics and foams. The PFAS compounds found in the cabin dust are also likely to reflect the use of certain waterproofing materials that were used at various points to prevent microbial growth in the ISS. Similarly, the elevated levels of BFRs could be a result of the astronauts’ practice of vacuuming the wall panels and acoustic insulation inside the ISS, which are likely to contain higher levels of these flame retardants.

“Our findings have implications for future space stations and habitats, where it may be possible to exclude many contaminant sources by careful material choices in the early stages of design and construction,” said study co-author Stuart Harrad, a professor of environmental chemistry at the University of Birmingham.

The concentration of the chemical contaminants found in the space dust was also compared against the median concentrations for these chemicals found in typical house dust in the United States.

“While concentrations of organic contaminants discovered in dust from the ISS often exceeded median values found in homes and other indoor environments across the US and western Europe, levels of these compounds were generally within the range found on Earth,” Harrad said.

Reference: Harrad S, Abdallah MAE, Drage D, Meyer M. Persistent organic contaminants in dust from the International Space Station. Environ Sci Technol Lett. 2023. doi:10.1021/acs.estlett.3c00448

This article is a rework of a press release issued by the University of Birmingham. Material has been edited for length and content.