unit37 Where Does Bad-breath Odor Come From 口臭是怎样产生

We all have offensive breath at one time or another. In most cases, offensive breath originates from bacteria in the mouth, although there are other more surprising causes.

Until a few years ago, the most doctors could do was to counsel patients with bad breath about oral cleanliness. Now they are finding new ways to treat the usually curable condition.

Bad breath can happen whenever the normal flow of saliva¹ slows. Our mouths are full of bacteria feeding on protein in bits of food and shed tissue. The bacteria emit evil-smelling gases, the worst of which is hydrogen sulfide (硫化氢).

Mouth bacteria thrive in airless conditions. Oxygen-rich saliva keeps their numbers down. When we sleep, for example, the saliva stream slows, and sulfur-producing bacteria gain the upper hand, producing classic “morning breath”.

Alcohol, hunger, too much talking, breathing through the mouth during exercise – anything that dries the mouth produces bad breath. So can stress, though it’s not understood why. Some people’s breath turns sour every time they go on a job interview.

Saliva flow gradually slows with age, which explains why the elderly have more bad-breath trouble than younger people do. Babies, however, who make plenty of saliva and whose mouths contain relatively² few bacteria have characteristically sweet breath.

For most of us, the simple, dry-mouth variety of bad breath is easily cured. Eating or drinking starts saliva and sweeps away many of the bacteria. Breakfast often stops morning breath.

Those with chronic³ dry mouth find that it helps to keep gum, hard candy, or a bottle of water or juice around. Brushing the teeth wipes out dry-mouth bad breath because it clears away many of the offending bacteria.

Surprisingly, one thing that rarely works is mouthwash. The liquid can mask bad-breath odor with its own smell, but the effect lasts no more than an hour. Some mouthwashes claim to kill the bacteria responsible for bad breath. The trouble is, they don’t necessarily reach all offending germs. Most bacteria are well protected from mouthwash under thick layers of mucus (粘液). If the mouthwash contains alcohol – as most do – it can intensify⁴ the problem by drying out the mouth.

1 saliva   

n.唾液，口水

参考例句：

• He wiped a dribble of saliva from his chin.他擦掉了下巴上的几滴口水。
• Saliva dribbled from the baby's mouth.唾液从婴儿的嘴里流了出来。

2 relatively   

adv.比较...地，相对地

参考例句：

• The rabbit is a relatively recent introduction in Australia.兔子是相对较新引入澳大利亚的物种。
• The operation was relatively painless.手术相对来说不痛。

3 chronic   

adj.(疾病)长期未愈的，慢性的；极坏的

参考例句：

• Famine differs from chronic malnutrition.饥荒不同于慢性营养不良。
• Chronic poisoning may lead to death from inanition.慢性中毒也可能由虚弱导致死亡。

4 intensify   

vt.加强；变强；加剧

参考例句：

• We must intensify our educational work among our own troops.我们必须加强自己部队的教育工作。
• They were ordered to intensify their patrols to protect our air space.他们奉命加强巡逻，保卫我国的领空。