Outdoor Air Cleaning Is Possible, but Only at High Cost

Removing pollution from the air around us may be technically possible, but the most effective systems demand heavy infrastructure and energy. That is the central tension in a review of outdoor air-cleaning technologies gathered by a Thai-Australian research team led by Jeevan Bhatta of Mahidol University.

Mechanical filtration systems and so-called smog towers are considered highly effective against fine particles. They actively draw in polluted air and pass it through filter materials. Under controlled laboratory conditions, these systems removed 97 percent of fine particulate matter. Their cost, however, lies in the infrastructure and energy required to operate them.

Other approaches reduce the burden but also deliver weaker cleaning results. Solar chimneys use air warmed by sunlight to push it upward through filters. Wet scrubbers use water droplets or films to capture particles. Electrostatic filters and ionization methods can also be highly effective, though they require considerable effort. For gaseous pollutants, photocatalytic building surfaces are of interest. Materials such as titanium dioxide use light to create reactive oxygen radicals that oxidize nitrogen oxides and volatile organic compounds. Biotechnology remains promising, but is still at a very early stage.

The review also points to a familiar, low-resource method: planting trees, hedges and other vegetation. Leaves and needles collect particles on their surfaces and can reduce pollution locally. The effect remains limited, but plants require no energy and also provide shade and improve the microclimate.

Progress in air pollution control has already come through fuel desulfurization, filters, electrification and other measures. Yet fine particles, nitrogen oxides, ground-level ozone and volatile organic compounds continue to pose a problem. The analysis therefore presents outdoor air cleaning as a possible supplement, not a substitute for preventing emissions.