A chlorine compound responsible for 10-15 percent of the ozone-depleting chemicals in atmosphere is still being emitted at rates that could affect the ozone layer which is still recovering.

Carbon tetrachloride (CCl4) used in the chemical industry is being emitted, despite a ban, at rates many times over those reported by industries in the US using them.

This could be due to underreporting of emissions from known sources or those from an unknown source or geographical process. The study, led by CIRES scientist Lei Hu and NOAA scientist Stephen Montzka, suggests that the source of the unexpected emissions could be associated with the production of chlorinated chemicals used to create Teflon and PVC. The new analysis is published in the Proceedings of the National Academy of Sciences.

If these industrial processes are in practice across the globe, it could explain the slow decline of CCl4 in the atmosphere, as also the slow recovery of the ozone layer. The United States has been responsible for about 8 percent of the overall global CCl4 emissions in recent years.

In the 1980s, this chlorine compound, as also the chlorofluorocarbons (CFCs) used in refrigerants, was placed in the list of substances to be phased out given its role in the destruction of the ozone layer. That list required that production of CCl4 for dispersive use should be discontinued in developed countries by 1996, and in developing countries by 2010.

"When we look at the amounts produced and destroyed, which industry throughout the world has reported to the Ozone Secretariat, we would expect the chemical's global concentration to be decreasing at a rate of nearly 4 percent per year. But it's only decreasing at 1 percent per year. So what's happening?" said Montzka.

Since the late 2000s, the team tracked the composition of the atmosphere from a network of nine tall towers and other aircraft-sampling sites across North America. They also looked at landfills, high-density population areas where use of bleach or chemicals in laundry or swimming pools could be the culprit, in addition to industrial sources for which the Environmental Protection Agency (EPA) had data on submitted emission figures.

Most of the emissions were from the area housing the industrial units. The amount found was 30 to 100 times higher than what was being reported.

The ozone layer at 15 to 30 kilometers above Earth acts as a shield from the harmful ultraviolet B radiation emitted by the sun. Even a 1% change in ozone layer can lead to a 4% increase in skin cancer.

Ozone is highly reactive molecule containing three oxygen atoms. It is constantly being formed and broken down in the stratosphere. The chlorine in CFCs, HClFCs and CCl4 in the presence of ultraviolet rays reacts with the ozone molecule and breaks it down. One atom of chlorine can destroy more than a hundred thousand ozone molecules, according to the EPA.

A hole in the ozone layer above Antarctica which was twice the size of the US was first detected 30 years ago. Research placed the blame on human influence and in particular on CFCs used in aerosol sprays and refrigerators.

The Montreal Protocol in 1987 banned all products containing CFCs. Since then, monitoring of the ozone hole has shown positive results and Nasa expects it to close over in a couple of decades. Fluctuation in the size of the hole, at times expanding like it did last year have been dismissed as normal phenomenon.

In 2014, the World Meteorological Organisation (WMO) first reported signs of ozone recovery while cautioning that it could be a decade before the hole begins shrinking. Many nations are still working on phasing out HCFC (hydrochlorofluorocarbons) gases used in cooling equipment, which also destroys the ozone layer. The latest report on high rates of CCl4 emissions is proof that much work still needs to be done in patching the ozone shield.