Smog is an ever-present urban problem hanging over many large cities today, primarily the result of car and truck emissions combining with industrial fumes. These emissions react in the atmosphere with sunlight, taking on a life of their own and often referred to as "photochemical smog."
In the United States, nine of the top 10 smog capitals are in California, with the Riverside-San Bernardino, Visalia-Tulare-Porteville and Los Angeles-Long Beach areas making up the top three. The lone top 10 U.S. smog capital outside of California is Houston, Texas. More than 1 million people die each year because of elevated levels of outdoor air pollution, and many more become ill, according to the World Health Organization. The American Lung Association recently reported that about 127 million Americans - more than 40 percent of the U.S. population - still live in areas that have been graded with an "F" in air quality.
To help address the issue, the commercial sustainability industry has brought to market a number of solutions ostensibly designed to combat urban smog with technology.
The latest one is from Pittsburgh-based aluminum giant Alcoa. The company claims that its new building coating literally eats smog. Called EcoClean, it will make its first appearance on a new regional electronics recycling hub for Electronics Recyclers International in Badin, N.C. According to Alcoa, the coating, which is pre-applied to the company's Reynobond aluminum panels, absorbs airborne pollutants and transforms them into harmless substances.
The original purpose of the coated aluminum panels, which were launched last year in collaboration with Japan's Toto (the world's largest producer of toilets), wasn't to improve air quality but to stay clean and bright, sloughing off smog stains, bird droppings and other city gunk that stain exterior walls. The positive effect on air quality was a secondary (but not insignificant) benefit, depending on the size and location of the building, according to Talking Points Memo (TPM).
Alcoa's research and development personnel claim that 10,000 square feet of the EcoClean-coated aluminum panels have the "smog-removal power" of about 80 trees, enough to offset the smog belched out by four cars per day.
How do the panels eat smog? With titanium dioxide (TiO2), which is used in everything from toothpaste to white paint. TiO2 is a photocatalyst under ultraviolet light, so when exposed to sunlight, its electrons energize, and upon contact with water vapor in the air, the oxidizers (free radicals) that are formed attack particles of organic matter laying on a surface, i.e., the gunk atop an aluminum panel, or floating in the air nearby. This leaves the organic matter barely clinging to the slick, super-hydrophilic surface of the panel and waiting for a good rain to be washed away along with most of the broken-down pollutants and bird poop. (Actually, it doesn't even need a hard rain; Alcoa says the wash-off effect occurs with drizzle, mist or just high humidity.) The result is a cleaner, better-looking building with lower maintenance costs.
Nitrogen oxide (NOx), a primary component of smog, is one of the contaminants rendered harmless by TiO2. The reaction between the titanium dioxide, ultraviolet light and water vapor transforms the airborne NOx into a nitrate that while perhaps is not ecologically neutral certainly is better for air quality. (Too much nitrate in groundwater has been tenuously linked to methemoglobinemia, or blue baby syndrome, in young infants, particularly in areas where there are high concentrations of nitrates from fertilizer in soil and groundwater.)
Titanium Dioxide Has a History
It's not the first time that TiO2 has been used to reduce smog. The commercial TiO2 smog-eating concept dates back to 2007, when Italian multinational cement and concrete company Italcementi Group developed a kind of anti-pollutant concrete for commercial buildings with a goal of keeping buildings cleaner.
Italcementi first developed the cement, called TX Active, for American architect Richard Meier, who at the time was building the Jubilee Church (Dio Padre Misericordioso ) in Rome, reported the New York Times. Meier wanted the exterior of the church to be - and to remain - blinding white, which was a tricky prospect in a city like Rome. The solution was exterior walls that contained a self-cleaning substance.
Italcementi tested the substance in Bergamo, Italy, where the company is headquartered. A stretch of road in the central part of the city was coated with a layer of TX Active. Residents soon began reporting cleaner air inside a 4.5-square-mile radius of the application. Italcementi says its research indicates that if 15 percent of the surface area of Milan were covered with the TX Active concrete, the city would see a reduction in smog of about 50 percent.
The substance is claimed to work particularly well in sunny cities, since the conversion effect is strongest when UV exposure is at the maximum. So the sunnier it is, the more dramatic the smog-eating effects. TX Active has also been used in the town of Segrate, Italy, near Milan, where Italcementi says it has reduced NOx by about 60 percent. The substance has also been used on Air France's new headquarters at Charles de Gaulle Airport, in Paris, and at the Hôtel de Police Commissariat building in Bordeaux, France.
Similar technology is being used elsewhere in the commercial sustainability industry, in everything from paint to concrete.
Pureti, a New York City-based company, has developed a TiO2 nanoparticle spray that dries into a clear coating and is designed to be used on a variety of surfaces, such as roads, rooftops, fabrics, building exteriors, solar panels and windows. Pureti claims that tests of TiO2-coated asphalt roads showed a decrease in smog-causing pollutants in their vicinity by about 50 percent.
One of the most polluted cities in the world, Manila, Philippines is using an air-purifying paint created with TiO2 to filter out nitrogen oxide when it reacts with sunlight and water vapor. The paint is being applied around the city, particularly on walls near highways. The catalytic paint, called Boysen KNOxOUT, appears to be making at least a small difference in the city's air quality, reports Grist. Commenting on the urban "smog-scrubbing" paint program, an environmental scientist recently told the BBC that the paint is capable of scrubbing out about 20 percent of smog in the air.
A roof tile manufacturer, MonierLifetile, is offering a product, called Auranox, that it claims is a smog eater, too. The product operates under the same catalytic and neutralization process. According to MonierLifetile, a 2,000-square-foot roof outfitted with Auranox tiles can eliminate the air the same amount of NOx that a typical car produces over 10,000 miles of operation. The Auranox panels first debuted in summer 2010, when Los Angeles-based home developer KB Home unveiled a prototype green home in Lancaster, Calif., that was outfitted with solar panels, renewable energy storage, LED lighting and an electric vehicle outlet besides smog-eating roof tiles.
TiO2: It's Not a Free Lunch
The sustainability question now is whether the use of titanium dioxide in building materials has a negative ecological and human-health impact. While the concern of nitrates in rain runoff is as yet not definitive -- many scientists say there's still no real answer as to whether increased nitrate levels have a significant health impact -- the concerns lie in the manufacture of TiO2 and in its physical form.
"The process of synthesizing titanium dioxide releases staggeringly huge amounts of toxic compounds such as chlorine gas, sulfuric acid, sulfur dioxide, sulfur trioxide and compounds that create smog itself, especially nitrous oxide," said John W. Rooney, a Ph.D. and J.D. in environmental law, and the curriculum development consultant for the Saylor Foundation, in Washington, D.C. In other words, the sustainable building industry may simply be shifting the smog from a TiO2-clad skyscraper to the location of the factory that manufactures the TiO2.
The second concern - the form that TiO2 takes - is significant, as well. Though titanium dioxide is a naturally occurring compound that has been determined to be safe by several regulatory agencies, the form of titanium dioxide most efficient at photocatalysis consists of networks of very fine nanocrystals, said Rooney.
"While the titanium dioxide we are familiar with in substances like paints, plastics, papers, inks, foods, cosmetics and toothpastes consists of large particles and poses no danger to human health, titanium dioxide nanoparticles have been shown in multiple studies to be both cytotoxic [leading to cell death] and carcinogenic," he said. Because of this, it's critical to determine how these TiO2 nanoparticles might degrade, and where they might end up when they do.
While nobody doubts the health benefits of cutting down smog in the world's dirtiest cities, the old environmental adage that "there's no such thing as a free lunch" applies here, and the issue deserves further study before anyone decides that it's a great idea to coat an entire city in TiO2 nanoparticles.