TEV, a trekking guide in Nepal, was looking extremely pleased with himself. He had just returned from a week-long trip which had earned him the equivalent of HK$207 - sufficient to keep him, his wife and young baby for a month. Tourism is one of Nepal's biggest currency earners, bringing in more than US$60 million in foreign exchange in 1992. But it's a money-spinner that comes at a high price. Mounds of plastic bottles and other non-biodegradable rubbish are left on the mountainsides by visitors each year. The nearest plastic recycling plant is in India and the process is too expensive for Nepal to consider seriously. Makeshift toilets add to the many water-borne bacterial diseases which kill tens of thousands of babies each year. Not for nothing are Nepal's most popular trekking routes known as ''toilet paper trails''. Other signs of environmental devastation are less obvious to the eye. ''Garbage is more visible'', comments Ukesh Bhuju of the World Wide Fund for Nature (WWF). ''But you have to go beyond the trails to see deforestation.'' The cause, though, can be seen immediately. Although tourists are no longer permitted to use firewood when camping, that does not prevent locals catering for them in the many tea lodges that line the paths. Ninety-six per cent of Nepal's energy comes from wood, but the country's forests are disappearing at a rate of three per cent a year. Tourism is responsible for a large part of this. ''The locals don't have electricity and kerosene is very expensive. It has to be carried from the nearest village, which in some cases could be up to seven days' walk,'' explains Yogendra Kayastha of the Kathmandu Environmental Education Project (KEEP). ''Instead they use wood for everything - building their houses, cooking food and heating water.'' The WWF estimates that the average tourist uses up to ten times the amount of wood a Nepalese needs. The organisation claims trekkers demand types of food which take much longer to cook than the Nepalese staple fare of lentils and rice, known as dahl-baht. It also says visitors are more likely than local people to want heating and hot water for showers. Some 334,000 people visited Nepal last year, and 80,000 went trekking. The majority of walkers head for the regions of Annapurna and Everest. In Annapurna the number of visitors equals the number of indigenous people there. Tourism is increasing by 17 per cent each year and the region is struggling to cope with the influx. Sagarmatha, or Everest, once called the world's highest trash pit, is suffering similar problems. Less accessible, it attracts fewer visitors, but the area being so remote - a four-day walk from the road - they tend to have an even bigger impact on the environment. In 1971 around 1,000 people travelled to the area. Today about 9,000 reach Mount Everest Base Camp each year, and some 40 expeditions tackle the mountain itself. Although tourism is the country's third biggest source of income, only a small amount of that cash filters down to village level. Around 80 per cent of all money brought in from tourism is spent on importing foreign goods, generally for the consumption of tourists. Kerosene is government-subsidised and costs about HK$4 a litre. But Ukesh Bhuju said: ''Most of the kerosene consumed is not for local food preparation or lighting, but for preparing food for trekkers in the restaurants. It's wrong, but many things that have been subsidised in Nepal for the local people end up beingused by tourists.'' Outside the towns, and aside from tourism, most Nepalese are subsistence farmers, living off the land. Theirs is a traditional life which tourism, bringing with it its Western influences, is beginning to erode. Begging is now a part of the get-rich-quick mentality which threatens rural life. Critics say families are being driven apart, the men leaving home at regular intervals to work as trekking guides or porters, the children and women staying behind to run the tea lodges. Efforts are being made to try to reduce the detrimental effect tourism is having on Nepal. Education is seen as the key. ''If you're trekking with a friend or a group, always ask for the same food at the same time, and help lodge owners save firewood,'' Yogendra Kayastha advises the up to 80 trekkers a week who visit KEEP's office in Kathmandu. ''And don't ask for a hot shower or boiled water for washing unless you are sure the lodge owner is using kerosene or solar power to heat the water.'' Local people are also targeted and KEEP offers eco-trekking workshops to teach camp site management, garbage disposal, first aid and hygiene to anyone working in the tourist industry. In Annapurna a conservation area has been established and a grassroots philosophy developed, which involves local people in all aspects concerning environmental protection and development. Han Gurung of the Annapurna Conservation Project Area (ACAP) says it is one of the most successful environmental protection schemes to be carried out in a Third World country. ''We are very specific in our aims,'' he explains. ''For example, to stop deforestation, people have to plant more trees. So we have set up forest nurseries and we provide seedlings free of charge. ''The people, they can take as much as they like and it won't cost them anything. ''To reduce the impact on firewood'', he adds, ''we have a strategy which we call the alternative energy development programme. Basically technologies, such as micro-hydro electricity or solar heating, can significantly help reduce the demand for firewood.'' Clean-up campaigns, loans which enable lodge owners to build toilets, research into local wildlife and the development of kerosene depots are all part of the project. In Annapurna local people are involved on a decision-making level. This, conservationists believe, is the way for the future. Although the first protected mountain area was established nearly two decades ago, it has taken many years more for a conservation programme to really take a grip in Nepal. But it will take the co-operation of all for it to have a long-lasting effect.