BioEd Online

Following explosions at two chemical plants in China, large amounts of toxic compounds have leaked into rivers that supply local populations with drinking water.

On 23 November local authorities suspended the water supply for the 3 million residents of Harbin in northeast China after an explosion at the Jilin Petrochemical plant, more than 350 kilometres upstream, spilled about 100 tonnes of chemicals into the Songhua river more than a week earlier. Thousands of people are reported to have fled the city in panic. Meanwhile, Russian officials in Khabarovsk have begun testing water in the Heilongjiang River, following warnings that the toxic spill could be carried across the border.

On 24 November, a second blast at Yingte chemical plant near the southwestern city of Chongqing released a huge yellowish cloud. Fearing benzene contamination there as well, Chinese officials ordered the evacuation of thousands of residents and told people to stop using water from taps.

News@nature.com takes a look at what happened and what it means for the people and the environment.

What kind of chemical plants were these?

The two factories are petrochemical plants, where various basic chemicals are isolated and extracted from crude oil and purified for industrial use.

How much toxic material have the explosions released into the environment?

Probably about 100 tonnes of benzene, and around 2 tonnes of other basic chemicals, including nitrobenzene, aniline and xylene, have been released at Jilin. The chemicals and amounts released at Yingte are not yet known, but a benzene spill is likely.

What can these chemicals do to people and the environment?

Prolonged exposure to benzene can cause cancer. Studies on workers regularly exposed to the chemical, such as shoemakers, have shown that it can cause leukaemia. It may also cause heritable genetic damage.

The clear colourless liquid is irritating to the eyes and skin, and may cause lung damage if swallowed. One teaspoonful of pure benzene is lethal. In smaller doses it can cause dizziness, headaches and nausea if inhaled.

Benzene is highly flammable, but not explosive. It does not stick around in water or soil because it biodegrades and evaporates rapidly. In the air it is broken down by sunlight.

Nitrobenzene can also cause cancer. Aniline is known to destroy red blood cells, and xylene has the potential to damage the central nervous system, liver and kidneys with long-term exposure.

Are the concentrations in this spill high enough to cause nasty effects?

The World Health Organization's guideline for tolerable amounts of benzene in drinking water is 10 micrograms per litre. According to Chinese officials the contamination of the Songhua exceeds this value by a factor of 100.

Toxicologists say that the increased exposure is tolerable for a limited period; it is unlikely to lead to a wave of disease or cancer, says Martin Göttlicher, head of the national German research centre for health and environment in Munich. But locals are certainly advised not to drink it, or even wash with it.

Is benzene a common chemical?

Yes, benzene (C₆H₆) is extremely common, and is present in almost every chemical factory around the world. More than 30 million tonnes of it were produced in 2000 worldwide. Production across Asia, except Japan and the Middle East, has recently been growing at an annual rate of 10%. Benzene is a basic chemical used to make numerous products including solvents, pesticides, paints and plastics. It is also found in petrol.

Could similar spills happen elsewhere?

Experts doubt whether a similar accident could easily happen in Europe or the United States.

In the European Union, plants with a storage capacity of more than 50 tonnes of dangerous chemicals, such as benzene, are classified as 'Seveso' plants (named after a 1976 dioxin accident in an Italian plant). Thousands of such plants exist in Europe, many of which use and store benzene in large amounts.

Seveso plants are subject to special safety requirements to prevent toxic substances from leaking, even in the case of major accidents. These requirements include safety valves and other blocking mechanisms that can stop the supply of chemicals when a fire or breach happens, and collecting tanks that prevent leakages into rivers and groundwater.

The amount released in China indicates that similar safeguards were not in place, says Göttlicher.

How can it be cleaned up, if at all?

If necessary, it might be possible to pump the benzene spill out of the water. But at normal temperatures, benzene is rapidly metabolized by microbes into harmless compounds. The worst contamination should be over within days to weeks, says Göttlicher. But in the meantime, millions have been left without water.