Some people think electronic cigarettes（e-cigs）， which don't contain tobacco， are a safer alternative to true cigarettes. But smoking e-cigs， or vaping， exposes people to toxic gases that can harm the lungs and cause other health problems. Now， a new study shows that the hotter an e-cig gets — and the more it's used — the more toxic compounds it gives off.

Hugo Destaillats is a chemist at Lawrence Berkeley National Laboratory in California. His team has just turned up a host of toxic chemicals in the vapors generated by electronic cigarettes. Some have never before been seen in vaping emissions. "There is this image that e-cigs are a lot better than regular cigarettes， if not harmless，" Destaillats says. As a result of the new research， he says， "We are now definitely convinced that they are far from harmless."

Key to vaping is the e-liquid that is drawn through an e-cig. Manufacturers dissolve flavorings， and usually nicotine， into this solvent. In most cases， this solvent is known as food-grade， meaning it should be safe enough to eat.

As a vaper inhales， some of the flavored solvent will flow through the e-cig. As it passes over one or more hot metal coils， that liquid vaporizes into a gas.

These flavored vapors can be a rich source of toxic chemicals. One of those toxic chemicals is nicotine. It is usually added to the starting liquid to stimulate a user's nervous system the same way that tobacco cigarettes will （Indeed， e-cigs are considered "tobacco products" by government agencies because their nicotine comes from tobacco plants）. But the new study finds that the solvents make even more chemicals that are bad to breathe in. As the solvents encounter an e-cig's intensely hot coil， they can break down to form new toxic compounds.

Those chemicals include formaldehyde and acetaldehyde. Both are considered likely to cause cancer. Another toxic aldehyde — acrolein — can severely irritate the eyes and airways.

The Berkeley team used two current types of e-cigs and three different e-liquids. Their test equipment mechanically drew air through the electronic devices. This created vapors that a user would normally inhale. The chemicals making up those vapors changed as the e-cigs heated up. That means the first puffs contained somewhat less of the toxic chemicals than later puffs did.

Some e-cigs， such as ones they used in these tests， allow a user to vary the voltage （battery power used to heat the metal coils）. Higher voltages produced hotter coils and more of the toxic chemicals.

1. What does "vaping" mean in the first paragraph？ ____.

A. tobacco smoking B. e-cig smoking

C. exposure to toxic gases D. inhaling harmful gases

2. What is the popular myth according to the passage？ ____.

A. Electronic cigarettes are a safe alternative to true cigarettes

B. Electronic cigarettes are harmful to people's health

C. Tobacco is harmful to peoples health

D. Passive smoking is harmful to people's health

3. What does "this sovlent" in the third paragraph refer to？ ____.

A. the e-liquid B. the flavorings

C. the nicotine D. the e-cig

4. Which of the following is NOT true？ ____.

A. The more an e-cig is used， the more toxic compounds it gives off

B. The first puffs are more toxic than later puffs

C. The hotter an e-cig gets， the more toxic it becomes

D. The later puffs are more toxic than the first puffs