Beijing is a city on the rise. The Chinese capital’s metropolitan area is home to 24 million people who are enjoying a development boom, courtesy of the city’s thriving economy (which grew 6.7 percent year-over-year in 2017, according to China Daily). For example, in 2000, Beijing had two subway lines with a total length of less than 55km; by 2012, the subway network included 17 subway and light rail lines with a total length of 456km—a figure which is set to nearly double again by 2020. As the middle class prospers, it’s moving away from the city’s historical mode of transport—the bicycle—towards the car as the primary source of transportation. In 2009, there were an estimated 3.5 million private autos; by 2017 that number had risen to around 6 million.
But this development has come at a price; the traffic and industry are poisoning the city. Beijingers talk of “airpocalypses” and “the dreaded PM2.5s” regarding the appalling air quality and 2.5µm in diameter atmospheric particulate matter (PM), which penetrates deep into the lungs and is deemed most hazardous to health. In late March, the South China Morning Post reported that the PM2.5 concentration was 238µg/m3 in Beijing, with a city average of 90µg/m3 over the year. World Health Organization (WHO) guidelines have stated that an average PM2.5 reading of just 25µg/m3 within a 24-hour period is unhealthy.
Worse yet, a 2016 report by Nanjing University’s School of the Environment concluded that 31.8 percent of all deaths in the polluted cities of China could be linked to PM2.5. While Beijing’s pollution is particularly bad, the city is far from alone in slowly poisoning its citizens. Los Angeles, California, is among the worst offenders in the US with an average annual PM2.5 reading of 18µg/m3. In Europe, Paris, France, recorded a peak reading of 55µg/m3 in February of this year.
While automobiles contribute to PM2.5 pollution, they are cleaning up their act, and other sources of pollution are becoming more significant. A 2018 report in Science revealed that consumer and industrial products now contribute fully half of the air pollution in 33 industrialized cities. The culprits are volatile organic compounds (VOCs), which are common constituents of plastics, paints, printing inks, adhesives, cleaning agents, and personal care products like hairspray and deodorants.
VOCs are defined as organic (carbon-based) compounds with a low boiling point and include hydrocarbons such as benzene (C6H6), toluene (C7H8), and xylene (C8H10), commonly used as solvents or precursors to other materials like plastics. Other common VOCs result from products such as terpenes, formaldehyde, and phenols. Some of these chemicals are quite nasty. Benzene, for example, has been shown to cause central nervous system and bone marrow damage and is carcinogenic.
According to the US Environmental Protection Agency (EPA), other VOCs can cause acute reactions such as allergies, headaches, loss of concentration, and worse. And it doesn’t take much of the substances to trigger these reactions. WHO guidelines suggest concentrations above 100μg/m3 (0.1ppm) trigger symptoms. The low boiling point of VOCs means that they rapidly turn into vapor at room temperature and can be easily sucked into the lungs.
While moving indoors protects against the pollutants from vehicle exhausts, it doesn’t reduce exposure to VOCs. That’s because buildings are built with VOC-emitting products, cleaned with VOC-emitting solvents, and are home to humans who are liberally sprayed with VOC-emitting personal products. Although a link hasn’t been proven, many believe that “sick building syndrome”—whereby building occupants experience detrimental health effects that appear to have a link to spending time in a building but where there is no identified cause nor specified illness—is a consequence of VOCs.
The design of modern buildings has exacerbated the problem. In the mid-1900s, building ventilation standards called for approximately 0.45m3/min of “outside air” for each occupant, primarily to “dilute and remove body odors.” As a result of the 1973 oil crisis, US energy conservation measures called for a reduction in external ventilation to 0.15m3/min. Insulation as well as heating, ventilation, and air conditioning (HVAC) systems of modern buildings have limited air exchange even further. Now, VOCs have nowhere to go.
A recent study linked VOC concentrations to office workers’ cognitive ability. A 2016 Environmental Health Perspective (EHP) study showed that when comparing cognitive performance at “normal” levels of exposure to VOCs at 500µg/m3, workers’ cognitive ability improved by 61 percent when concentrations were lowered to less than 50µg/m3 and by 100 percent when the VOC pollution was less than 44µg/m3.
Perhaps not surprisingly, employers are sitting up and taking notice of worry over VOCs. Concern for workers’ health is one incentive, but potential productivity losses is perhaps a greater one. Such concerns have led to the commercial development of compact, inexpensive, battery-powered VOC monitors. The metal-oxide semiconductor (MOS) VOC sensor is proving to be the most popular for building applications because its detection range matches the likely concentrations of VOCs in indoor workplaces.
On contact with a target VOC, the sensor’s conductivity or resistivity changes from a known baseline value. The change in the sensor’s electrical characteristics is typically a linear and proportional relationship with the VOC, enabling a simple calibration to determine the pollutant’s concentration.
The installation of MOS VOC sensors is rapidly being considered a routine inclusion, together with temperature-, humidity-, motion- and smart lighting-sensors, in the construction of smart buildings. The sensors monitor the environment and send their data to supervisory computers via the Internet of Things (IoT) connectivity. The data enables the supervisory systems to keep the building comfortable, energy efficient, and, above all, safe.
According to analyst MarketsandMarkets™, by 2023 the VOC sensor market is expected to grow to $1.3 billion, driven by the enforcement of occupational health and safety regulations by the US government, by the development of miniaturized wireless sensors, and by the increasing awareness of the benefits of air quality among the public.
Steven Keeping gained a BEng (Hons.) degree at Brighton University, U.K., before working in the electronics divisions of Eurotherm and BOC for seven years. He then joined Electronic Production magazine and subsequently spent 13 years in senior editorial and publishing roles on electronics manufacturing, test, and design titles including What’s New in Electronics and Australian Electronics Engineering for Trinity Mirror, CMP and RBI in the U.K. and Australia. In 2006, Steven became a freelance journalist specializing in electronics. He is based in Sydney.