PaUL POLMaN is used to feeling uncomfortable. The 54-year-old ran the New York marathon in November and climbed Kilimanjaro in 2005. The challenge that is causing the Dutch-born chief executive of Unilever most pain, however, is the consumer-product giant's plan to go green.  The Sustainable Living Plan aims high. Unilever, which makes 400 of the world's best known household goods, including Persil, Ben & Jerry's and Dove, wants to halve its impact on the environment over the next decade - and double sales at the same time.  "It's very ambitious," said Polman, "and it makes us feel a bit uncomfortable. We are creating a new model that will decouple growth from overall impact on society."  The growth part of the story - doubling its 2010 sales of ?39.8 billion (?34.4 billion) - appears to be the easy bit.  at least, it's the part causing Polman least discomfort. He expects to attract another 1 billion customers in the next decade from developing countries.  "There are people in many parts of the world who aspire to our standard of living. But growth at any cost is not viable," he said.  The plan has been two years in the making. It was started by Polman, who joined from Nestl? in January 2009. So far Unilever has been measuring the effect its products have on the environment - no easy task, as the group supplies thousands of individual products and deals with hundreds of suppliers.  The analysis covered the life cycle of the products. "Most companies just stick to their own shop and say 'hey, I've reduced carbon output in my factory by 30% or I've turned the lights off in my office'," said Polman, "but we look at the total impact."  Unilever's own operations - manufacturing and distribution - account for only about 5% of the total environmental damage. Most of it happens at the beginning of the cycle, in the way raw materials are sourced, and at the end, in how people use and dispose of products. For example, 60% of the greenhouse gases emitted in the life cycle of a cup of tea occur when the consumer boils the kettle. To reach his target, Polman needs to get people to switch to energy-efficient kettles and to boil just enough water for one cup.  Personal hygiene presents even greater challenges. In the West, people can use up to 20 litres of water each time they wash their hair, which means more than 95% of the water usage of a bottle of shampoo is in the hands of the consumer.  On top of that, 70% of its greenhouse gas emissions come from the hot water needed for its soaps, shower gels and shampoos to work.  Surely, Polman can't start telling us to spend less time in the shower? "You can change people's behaviour," he said.  "Remember it was quite normal to drive without a seatbelt or have kids in the front seat. It wasn't just the law that changed, people realised it was dangerous. Increasingly in homes there is an awareness that excessive water usage is not socially acceptable behaviour."  The company has form in changing consumer habits. Polman points to the Cleaner Planet Plan, which educated the public about the benefits of washing clothes at lower temperatures and using shorter spin cycles.  To get anywhere near achieving the ambitious objective of doubling sales while halving environmental impact, Unilever needs to start providing customers with hygiene solutions that require less water and energy. It aims to reach 400m people with these products by 2020. But there is still a long way to go. The report admits: "We do not yet know how we will do this."  Polman, howeve