Burning Issues


  • Demonstrate that particulate air pollution can be invisible to the eye.
  • Discover various sources of particulate air pollution and think of solutions for preventing air pollution.


  • Clean tuna can (or similarly shaped), top off, bottom on, or a candle holder
  • Matches, glass of water
  • Votive (normal size) candle
  • White or clear pyrex bowl or other heat resistant glass
  • Optional hand lens or dissecting microscope


  1. Ask students whether they think pollution is always visible? Can it be invisible? Discuss their answers and tell them that they'll be watching a demonstration that shows you can't always see air pollution.
  2. Place the candle where everyone can see it and light it. Have a glass of water handy to extinguish the match quickly so students are not distracted by smoke from the match.
  3. Ask the students if they can see any pollution coming from the burning flame.
  4. Next, lower the bowl over the candle until its bottom barely touches the tip of the flame, hold it there for a few seconds. Move the bowl away from the flame, leaving the candle burning.
  5. Have the students inspect the soot that has collected on the bowl and ask them for ideas about where it came from. The burning candle releases gases and very small particles of the burned wax into the air, making air pollution. The pollution, however, wasn't visible until it was collected on the bowl.
  6. Have the students identify things that might release similar, very small particles into the air, as the candle did. (Ideas: cars, trucks, buses, forest fires, open burning, the family barbecue.)
  7. Have the students identify alternatives that release less of these particles of pollution, or none at all, such as bicycles, electric/hybrid cars and electric or push lawn mowers, solar heated houses, etc.
  8. Inspect the individual soot particles with a hand lens or dissecting microscope. Discuss how larger particles are kept or cleared out of our lungs by our bodies' natural defenses (coughing, sneezing, the action of "cilia" (hair-like structures) in the airways) but that smaller particles like those the candle produced get lodged in the lungs and stay there, causing harm. Another example would be the effects of smoking.