In 2012, seven million people around the world died from breathing dirty air, the World Health Organisation noted as it declared atmospheric pollution to be the planet's greatest environmental health risk. Astonishingly, an estimated 4.3 million lives were lost in 2012 because of exposure to indoor air pollution.

Grimy air inside homes is just as strongly linked to cardiovascular diseases, respiratory diseases and cancer as exposure to outdoor pollution, said the WHO study.

Most studies blame indoor air pollution on the smoke and particulate matter emitted by fuels such as coal, wood or crop residue used for cooking in rural areas and urban slums. But even in an average urban household that uses cleaner fuels in the kitchen, the air is filled with hazardous pollutants emitted by everything from the paint on the walls to the polish on furniture and the foam in mattresses.

“In urban homes, chemical exposure is a big challenge because the exposure can be both, through inhalation or through touch,” said Chandra Bhushan, deputy director of the Delhi-based research and advocacy organisation Centre for Science and Environement.

The biggest and most dangerous chemical is, unarguably, lead. “For a long time, lead has been used in household paint, and lead particles penetrate the indoor air as paint peels off,” said Ravi Agarwal, director of Toxics Link, a non-profit organisation that spreads awareness about toxicity in the environment.

Breathing in lead – particularly for children and pregnant women – can result in stunted growth, kidney and nervous system damage and possibly cancer. It can also permanently reduce a child’s IQ.

“The WHO has stated that with lead, no amount can be considered as a safe level, because even 5 milligrams of lead in one decilitre of blood can be harmful,” said Agarwal. This is worrying, he says, because paint companies in India only began to make lead-free paints last year after sustained campaigns by activists. Since getting one’s home painted is an expensive, long-term affair, most people tend to have a lot of old paint on their walls, putting them at prolonged risk.

Another metal, almost as toxic as lead, is mercury – present not just in thermometers and healthcare instruments, but also in phosphorescent lights, batteries, soaps and cosmetics. The liquid metal can vapourise when it is exposed to air, and chronic exposure to it can affect the nervous, digestive and immune systems.

While CFLs – compact fluorescent lamps – are a great energy-saving option for lighting up homes and offices, Agarwal says the mercury content of CFLs in India is very high, because of the lack of any regulatory framework to standardise its use. In October, 140 countries signed the United Nation’s Minamata Convention, a treaty that aims to reduce the use and emissions of mercury across products. India was not a signatory to the treaty.

Number three on Agarwal’s list of household chemical pollutants are volatile organic compounds – VOCs – emitted from cleaning agents, varnishes, carpets, air fresheners and even upholstery fabric.  The concentration of VOCs tends to be high indoors, particularly in closed, air-conditioned rooms. Long-term exposure can damage the endocrine system and increase the risk of cancer.

“Most of the time, we don’t know what chemicals our cleaning agents are made of – even foam-based mattresses and wrinkle-free clothes contain hydrocarbons,” said Bhushan. In most countries around the world, companies are required to register data for toxicity in their products with some government agency. “But in India, you can sell any chemical-based goods because we don’t have chemical registry.”