Read the passage and answer the question. "During the dry season many Thai cities are covered by a haze of fine dust known as PM2.5. These particles are so small that they pass through the nose and travel deep into the lungs and even into the blood. Doctors advise people with asthma to stay indoors on days when the air quality index is red. A properly fitted N95 mask can filter out most of these particles, but a loose cloth mask cannot. An air purifier helps only if the doors and windows are kept closed." According to the passage, why is a cloth mask not recommended on days with heavy PM2.5?