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Indoor air pollution: the invisible adversary

  • The battle to curb ambient air pollution has been in the news for a long time
  • The less-researched hidden air pollution in homes, offices and other indoor environments is no less dangerous for your health

For the HOMEChem project, researchers deployed sensors and air quality monitors to make observations on change in air quality during regular activities such as cooking.
For the HOMEChem project, researchers deployed sensors and air quality monitors to make observations on change in air quality during regular activities such as cooking. (Photo: Callie Richmond)

Activist organization Centre for Science and Environment’s (CSE’s) air pollution analysis, released in August, holds bittersweet news for Delhi. It says the three-year average levels of PM 2.5—atmospheric particulate matter with a diameter of less than 2.5 microns—during 2016-18 were 25% lower than the 2011-14 baseline (three-year average). Though the number of days with severe PM 2.5 levels has come down since 2015, the CSE says Delhi still needs to cut pollution levels by 65% to meet global air quality standards.

While the battle to curb ambient air pollution has been in the news for a long time, the less-researched hidden air pollution in homes, offices and other indoor environments is no less dangerous for health. Not only are ambient air pollution and indoor air pollution inextricably linked, globally, people spend 90% of their time indoors.

According to the State Of Global Air Report 2019, published earlier this year by the Boston-based Health Effects Institute, an independent global health and air pollution research organization, an estimated 846 million people in India were exposed to household air pollution in 2017. That’s 60% of the country’s population.

A huge chunk of this number is said to consist of people living in rural areas, where coal, wood, charcoal, kerosene, crop waste and other types of biomass are still used for cooking, heating and lighting. Urban India isn’t much safer either. And yet there isn’t even a standard to measure indoor air pollution.

Let’s start with the kitchen. While a modern kitchen doesn’t use solid fuels, even gas burners and cooking methods affect air quality. A June 2018 study and experiment conducted by researchers at the University of Colorado Boulder (CU Boulder) found that “cooking, cleaning and other routine household activities generate significant levels of volatile and particulate chemicals inside the average home, leading to indoor air quality levels on par with a polluted major city". Much of this would be relevant in an Indian setting too.

The experiment, called HOMEChem (House Observations of Microbial and Environmental Chemistry), was conducted over four weeks and involved the use of advanced sensors, instruments, air-quality monitors and cameras to analyse the indoor air quality of a 1,200 sq. ft manufactured home on The University of Texas at Austin campus. This home set-up is one of the very few facilities dedicated exclusively to research on indoor environments.

According to a CU Boulder release, over the course of a month, Marina Vance, an assistant professor of mechanical engineering, and her colleagues “conducted a variety of daily household activities, including cooking a Thanksgiving dinner in the middle of the Texas summer". Vance led the HOMEChem experiment along with Delphine Farmer, an associate professor at Colorado State University’s chemistry department.

Some of the questions that the experiment tried to answer included tracing the sources of chemical oxidants in the indoor environment, and how they are affected by human activities and changes in light conditions. The experiment also looked at the main sources of organic compounds and how these react physically and chemically, transforming from the gaseous state to the particulate phase—known as secondary organic aerosols.

What are you breathing?

Gases and particles in the air can be divided into two categories: primary and secondary sources. According to an overview paper on the experiment (Overview of HOMEChem: House Observations Of Microbial And Environmental Chemistry), which was published in the multidisciplinary journal Environmental Science: Processes & Impacts in July, primary gases and components are emitted directly from sources, whereas secondary species are produced through chemical reactions in the air.

In a typical indoor environment, primary sources would include “the building itself (e.g. wood, linoleum, plastics), consumer products (e.g. personal care products, cleaning or cooking products, equipment and office products, off-gassing from items brought into the home), microbial and human metabolic emissions, occupant activities (e.g. cooking)", and entry of outdoor air into the house, through openings, ventilation systems or leaks.

The paper explains how cooking turned out to be “a large source of VOCs (volatile organic compounds), CO2 (carbon dioxide), NOx (nitrogen oxide) and particles". Both VOCs and NOx are chemical precursors that react in the presence of sunlight to form ozone. Ground-level ozone not only has long-term effects on human health but is also a critical pollutant in smog.

“Let’s assume someone is staying near a major road. Vehicles are one of the strongest emitters of NOx. The emission of NOx has increased drastically after (the introduction of) CNG in Delhi. How much of that NOx infiltrates your house? If you have NOx and if you have VOCs, they will combine to form ozone. There is hardly any information on these reactions that can take place in an indoor setting," says Sagnik Dey, associate professor at the Centre of Atmospheric Sciences, Indian Institute of Technology (IIT), Delhi. Dey is also a coordinator at the Centre of Excellence for Research on Clean Air, a policy think tank and research centre established by IIT,Delhi in February 2018.

According to a paper published in April 2015 in the journal Current Sustainable/Renewable Energy Reports, “ozone also reacts with fragrance molecules, tobacco smoke residues, and even human skin oils to generate a host of oxidized organic compounds, secondary organic aerosols, and irritants".

In a more recent update, researchers at Pennsylvania State University used a computer model of indoor environments to show how “some volatile and semi-volatile gases and substances are produced when ozone reacts with skin oils carried by soiled clothes". According to the researchers, this reaction can produce a “personal cloud of pollutants".

Stir-fried pollution

In an article in The New Yorker earlier this year, New York-based writer Nicola Twilley notes how researchers and scientists in the HOMEChem experiment “began to layer activities together" once the baseline emissions for things like “cooking a stir-fry" had been identified. “A group of volunteers would spend the day in the house, cooking breakfast and lunch, checking their e-mail, cleaning up, making dinner, and running the dishwasher, in order to see whether, say, the emissions from frying vegetables in teriyaki sauce would react with the bleach fumes from mopping the kitchen floor afterwards. (Delphine) Farmer told me that, based on her preliminary data, it seems as though they did, producing temporary spikes of chloramines, a class of chemicals that are known to inflame airway membranes. Another product of the marriage of bleach-based mopping and gas-burner ignition is nitryl chloride, a compound that is known to atmospheric chemists for its role in coastal smog formation. No one had expected to find it indoors," writes Twilley.

Imagine the aerosols that could be created here if the emissions from the dish you just tempered or stir-fried reacted with the particles and gases from the commercial bleach solution or disinfectant you just used to wipe your kitchen slab. The burning of mosquito coils, common trend in Indian homes, and smoking in a closed indoor environment (say, your living room) can emit higher respirable particulate matter, which can lead to illnesses on prolonged exposure.

Indian researchers and academics say much needs to be done to ascertain the levels of indoor pollution and its sources. Pallavi Pant, a staff scientist at the Health Effects Institute, says household air pollution needs to remain a focus for policy action, especially in Asia and Africa, where the use of solid fuel for residential cooking and heating is still very high.

“Recent studies have also started estimating the contribution of household air pollution to ambient (outdoor) air pollution…. There are two main aspects at play here: first, the issue of solid fuel use in homes and second, the issue of exposure related to cooking, smoking, use of incense and/or mosquito coils, consumer products, including cleaning and personal products, infiltration of outdoor air etc," explains Pant on email.

“The latter is more relevant in urban households, offices and public spaces but has not received as much attention. Globally, there is a growing interest in understanding these sources of indoor exposure to air pollutants. In India, some researchers have started looking at this aspect but much remains to be done," she adds.

Pant lists some simple measures people can take to keep a check on indoor air pollution in an urban setting. These include cooking with appropriate ventilation (especially when frying food), ensuring there is no mould in the house, avoiding the use of incense sticks and candles, room fresheners, etc., where possible, and restricting the infiltration of outdoor air, especially on days when pollution levels are high. “It is important to mention that buildings in India often have natural ventilation and tend to be leaky. Special care is needed to fully seal the building," says Pant.

Dey says a big gap is the lack of a standard to measure indoor air pollution. “At least there is a standard for ambient air pollution. For instance, for PM 2.5, it is 40 microgram per cubic metre at an annual scale and at the 24-hour scale, it’s 60 microgram per cubic metre," explains Dey. “What we are measuring and what we are targeting (for indoor settings) is not clear at this moment. But from whatever data is available right now, in any natural house in India, indoor air is as bad as outdoor air."

Trouble in your heaven: The most harmful chemical compounds usually present indoor

Ozone (O3): Exposure to ground-level ozone increases a person’s likelihood of dying from respiratory disease, specifically cardiovascular disease

PM 2.5: In 2017, exposure to PM 2.5 was the third leading risk factor for type 2 diabetes-related deaths and disability

Nitrogen oxide (NOx): NOx can cause inflammation of the airways. Long-term exposure can diminish lung function and increase susceptibility to allergens

Cigarettes, mosquito coils: Particulate matter from burning in a closed indoor environment can lead to illness (cardiovascular and respiratory diseases) following prolonged exposure

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