Air pollution is a major public health issue worldwide. Filters can help improve the quality of the air we breathe, but they also contribute to landfill when they are finished with and thrown away, as they are often made of plastic. Could bio-based filters be the answer?
Researchers in the US have developed bio-based air filters that they claim can capture toxic chemicals that current filters can't. The scientists, at Washington State University (WSU), say that these inexpensive soy filters could lead to better air purification and improve air quality.
Air pollution causes an estimated 467,000 premature deaths every year in Europe, according to the European Environment Agency (EEA).
The agency's Air quality in Europe — 2016 report found that 85% of people living in urban areas in the EU are exposed to fine particulate matter (PM2.5) at levels deemed harmful by the World Health Organization (WHO). These tiny particles, which have a diameter of less than 2.5 micrometers, are considered some of the most dangerous air pollutants.
'There is evidence that shows that premature mortality is linked with air quality going back to the London smogs in the 1950s,' says Ian Colbeck, an environmental scientist at the University of Essex, in the UK.
Poor air quality isn't only linked to respiratory diseases like asthma, lung cancer and COPD. For example, there are reports showing that 'particulate matter can go through the lungs into the blood stream and cause strokes', and that it 'can get into the brain via the nostrils', causing diseases like Alzheimer's, explains Colbeck.
Air filters can be fitted to items such as air conditioning units (in vehicles and buildings) to filter air before it is inhaled. They can also be installed at the point of production - for example, on factory chimneys and car exhausts - to capture pollutants before they are released.
Most air filters consist of a mesh of plastic fires that physically filter particulate matter - PM2.5 and larger PM10 particles. But these particles are not the only pollutants of concern.
The EEA found that in 2014 96% of the EU urban population were exposed to ground-level ozone (O3) concentrations above WHO guidelines. Exposure to benzo[a]pyrene, sulphur dioxide (SO2) and nitrogen dioxide (NO2) was also high, with 88%, 38% and 7% of urban dwellers, respectively, living in areas with levels exceeding WHO recommendations.
Although 'the emissions of pollutants have in general decreased in Europe' there are still 'exceedances of the regulated concentration standards for almost all pollutants', explains the EEA's project manager for air quality reporting and assessment, Alberto González Ortiz.
The health impacts of this are significant. Within the EU, annual premature deaths linked to PM2.5 exceed 430,000, while NO2 is associated with around 71,000 early deaths and O3 an estimated 17,000.
Plastic filters are not able to capture these gaseous chemicals, unless other agents such as activated carbon molecules are added. However, test have shown that the soy protein filters can capture chemicals such as carbon monoxide, SO2 and formaldehyde.
Katie Zhong, lead researcher, and mechanical and materials engineer at WSU, explains that this is because 'proteins contain many types of functional groups and different functional groups can interact with different of toxic chemicals'.
To make the filters, a chemical process is used to disentangle the soy protein and produce nanofibres that can be spun to produce a protein-fiber mat - the filter.
Air filters have to be replaced regularly. 'Any filter is only as good as its maintenance,' explains Colbeck. As the soy filters are made from plant material they are biodegradable, unlike plastic filters, which can create secondary pollution when thrown away.
According to Zhong, another advantage of the soy filters is that they have a lower air resistance than conventional air filters. This means that less energy is required to pump air through the filters. This is particularly important for the filtering of chemicals as the addition of elements like activated carbon increases the air resistance of plastic-based filters.
The researchers are also experimenting with protein filters produced from other sources, such as gelatine. Zhong says that while these filters might have slightly different functional groups, enabling different filtering properties than soy-based filters, she thinks the 'bigger difference will be in the processing and some other properties, like thermal and moisture resistance'.
Zhong and her team are currently in talks with pharmaceutical and steel plants in China. Zhong says that these companies are interested in protein filters as there are not many materials that can effectively trap the toxic chemicals they release. But first they have to work out how to scale up the filters to a much larger size - something the researchers are working on.