Dumb eco-questions you were afraid to ask

New Scientist offers the definitive guide to everything you wanted to know about being green but were too embarrassed to ask

Switching the light on and off does saves energy, but there is a catch. Every time you flip the switch, the bulb takes a jolt of electricity, which shortens its life. Studies by the Electric Power Research Institute in Palo Alto, California, found that turning low-energy compact fluorescent bulbs on and off at frequent intervals can shorten their lifespan by as much as 75 per cent. The institute's director of energy utilisation, Tom Reddoch, suggests leaving energy-saving bulbs on if you will be out of the room for less than 15 minutes.

According to the Waste and Resources Action Programme (WRAP), based in Banbury , UK , pizza boxes are often not recyclable. That's because grease from the toppings contaminates the cardboard, making it useless to paper mills - though it can still be composted. Such impregnation is not a problem when it comes to cans and bottles. Nevertheless, they should be rinsed to remove food remnants so as not to attract vermin. Plastic should also be clean, and lids removed from bottles so they can be squashed flat. WRAP recommends rinsing waste items in old washing-up water to save energy.

Smoothly. Avoid dramatic braking and acceleration and use cruise control if you've got it. Move through the gears as quickly as possible, changing up before you hit 2500 revs per minute (2000 rpm for a diesel). Where possible, drive at a steady 55 miles per hour (90 kilometres per hour). It is up to 20 per cent more fuel-efficient than driving at 75 mph. Check your tyre pressure once a month because underinflated tyres can raise fuel consumption by 6 per cent. Don't carry excess baggage. Each extra 25 kilograms decreases fuel efficiency by 1 per cent. And avoid short trips - a cold engine uses twice as much fuel as a warm one.

It's unlikely. In May, Phil Edwards and Ian Roberts from the London School of Hygiene and Tropical Medicine did link obesity and global warming (The Lancet, vol 371, p 1661). They argued that obese people consume around 18 per cent more calories than the average and their greater mass means their vehicles require more fuel, so policies to encourage walking and cycling would not only improve people's health but also be good for the planet. Perhaps such policies would provide short-term benefits, but in the long run they would be more than offset by the fact that people who stay in shape are likely to live longer, emitting tonnes of CO₂ for every extra year of their lives. So being slim may be good for you, but is unlikely to benefit the planet.

According to the UK-based Environmental Transport Association (ETA), the most efficient conventionally powered cars are slightly less detrimental to the environment than hybrid models. However, it points out that the current crop of hybrids won't evolve without customers willing to invest in what is still frontier technology.

There are three main uses. Treated aerobically, organic waste is composted to produce soil conditioner or landscaping mulch, returning carbon to the soil. Under anaerobic conditions, it can be digested by bacteria to produce methane, used to generate electricity. Through a combination of biological and mechanical processes, it is also turned into fuel that can substitute for coal or coke in power stations or cement kilns.

Unless you are certain you won't have to pull away at short notice, the inconvenience and tiny amounts of fuel involved make stopping and restarting the engine hardly worthwhile. The UK Automobile Association (AA) recommends switching off if you are likely to be stopped more than 3 minutes. Technology is already providing a better solution, though. Recognising that many hours of urban driving are spent at a standstill, several car manufacturers have started to introduce so-called "stop-start" technology. In fact, Volkswagen recently mothballed plans for a hybrid car, preferring to invest in stop-start technology on its standard models to achieve fuel efficiency savings of between 10 and 15 per cent in urban traffic.

According to the ETA, when the average new car leaves the showroom, its manufacture, design and marketing have accounted for up to 6 tonnes of CO₂ emissions. "Nevertheless, swapping a thirsty and polluting older car for a lighter, more fuel-efficient model makes environmental sense," says Yannick Read of the ETA. But, he adds, gains made from exchanging a five-year-old family car with a similar-sized, brand-new model are likely to be negligible.

In energy terms you use the same amount of electricity either way. However, charging a battery little and often reduces its capacity to hold charge, and eventually it will lose its capacity to charge altogether. If you kill your battery you'll need a new one - and batteries take a lot of energy and materials to produce. So charge your battery fully and let it run down completely to get the longest possible life out of it.

You are much better off cycling. A 12-kilometre round commute on a bus or subway train is reckoned to generate 164 kilograms of carbon per commuter per year. Somebody cycling that distance would burn about 50,000 calories a year - roughly the amount of energy in 22 kilograms of brown bread. A kilo of brown bread has a carbon footprint of about 1.1 kilograms, so switching from public transport to a bike saves about 140 kilograms of carbon emissions per year. Although this only really works if enough people cycle to allow public transport providers to reduce the number of buses and trains they run.

Not if the car is also full. Consider this. EasyJet, which claims to be 30 per cent more fuel efficient than other carriers, largely because it packs in more people, calculates that on an average flight each passenger accounts for 95.7 grams of carbon dioxide per kilometre, compared with 164 grams for someone travelling by car. That flight will be around 80 per cent full, so the figure would fall to 76 grams per person if every seat were taken. What's more, most EasyJet flights are either short or medium haul, making them one-third less efficient than long-haul flights (over 4000 kilometres). Long-haul flights could bring the figure down to around 50 grams per passenger. However, using EasyJet's own figures, a full car would produce just 41 grams of CO₂ per kilometre for each of its four passengers. So cars win no matter what the distance - although clearly planes have the edge when travelling over water.

No. And that applies even if the plug is switched on or if the socket has no on/off switch. The exceptions are appliances with a standby mode, which include most battery chargers. As a rule of thumb, if there is a light on, a clock ticking or the transformer feels warm, it is using electricity. And that can be a substantial proportion of the amount the appliances consumes when in use. A television set-top box, for example, uses around 18 watts while it is on and almost 17 watts on standby.

No, absolutely not. According to Alcoa, the world's third-largest aluminium producer, manufacturing a can from recycled aluminium uses only 5 per cent of the energy of making one from scratch - an energy saving that could power a 100-watt bulb for 4 hours.

Over a 75-year lifespan, the average European will be responsible for about 900 tonnes of CO₂ emissions. For Americans and Australians, the figure is more like 1500 tonnes. Add to that all of humanity's other environmentally damaging activities and, draconian as it may sound, the answer must surely be to avoid reproducing.

The frost in your freezer forms when warm air condenses and freezes on the cooling coils. Self-defrosting fridges generally use more energy than manual models because their coils are automatically heated every few hours to melt any frost that forms on them. But a manual-defrost fridge is only better if you defrost it before the frost starts to take over, because the coils have to work harder to cool the air if they are covered in ice. How often you need to defrost depends on how often you open the door and the humidity of the air. As a general rule though, you should defrost when the ice gets to 5 millimetres thick. The best way to do this is to turn off the freezer, put pans of hot water inside to speed up melting, and then remove the big bits of ice with a plastic scraper.

Hands up everyone who thinks the small, round symbol of two arrows circling each other means that packaging can be recycled. Wrong! In fact, this so-called "green dot", or "grüner punkt" - which is often printed in black - originated in Germany and indicates that the manufacturer has paid into a scheme to meet the general costs of recycling under the terms of European Union legislation. So particular packaging bearing the logo may or may not be recyclable. In countries such as the UK that have not adopted the system, the logo is especially perplexing, as its use appears quite arbitrary. "We typically deal with multinational suppliers that often sell the same product in multiple countries and therefore include things on packaging that may be irrelevant in one of those markets," explains Katherine Symonds of the supermarket chain Tesco. Recognising that this can lead to confusion, she says Tesco has now established a working group with other retailers to make labelling "clearer and more intuitive".

Many people choose paper over plastic, figuring that being renewable, degradable and recyclable, it is probably the greener option. In reality it's not quite that simple. Paper is heavier and bulkier to ship than plastic, takes more energy to produce and uses damaging chemicals in its manufacture. Overall, the best packaging choice is the one that has the least total impact over its life - from raw materials and shipping emissions to toxicity and waste - and that depends on where it comes from and what you plan to do with it. Consumers often don't have the required information to work out this trade-off but there are some things to keep in mind. Check whether the paper or plastic has already been recycled, and whether you can reuse or recycle it. Also, if you tend to avoid packaging altogether, consider this: if it reduces the chances of a product perishing or breaking before it can be used then it is almost always better to have packaging than not. Just choose products with the smallest amount possible.