阅读下面材料，在空白处填入适当的内容（不多于3个单词）或括号内单词的正确形式。
Nowadays, many cities are suffering from smog, the short form of smoke and fog. It is especially harmful to children, seniors, pregnant women, smokers and people      lung or heart problems.
An important component (成分) of smog is ground-level ozone (地面臭氧). Actually, ozone that is formed naturally in the atmosphere      (protect) life from the sun’s damaging rays. At ground level, however, ozone hurts people’s eyes, nose, and throat, which can make      more difficult for the body to fight against an      (infect). When the amount of ground-level ozone increases, so does      number of emergency-room visits.
The other component is fine particulate matter (细颗粒物). The      (small) the particle (particulate matter 2.5, PM2.5), the more dangerous it can be. When people take in ozone and particulates from the air, both the blood flow      the oxygen supply to the heart will be reduced.

With such unhealthy air quality, some simple ways are recommended for you    (reduce) higher risk of health problems. On smoggy days, spend less time outdoors. If you have to get outside, close all your windows       you go out. Besides, roll up the windows of     car if you are driving.