It happens that I'm taking some medicine at the moment. 100 mg of one, and 10 mg of another. To make the math easy, let's say I weigh 100 kg. mg is milligram, so there are 1000 of them per gram. 1000 grams is 1 kg.
That first medication represents 1 part per million. The second is a mere 0.1 parts per million. The body is a complex system. Small amounts of things can be very important. Climate is also a complex system.
Let's continue a little in this vein. Daily nutrition requirements, which I'll take from http://en.wikipedia.org/wiki/Reference_Daily_Intake , include:
- 1000 mg Calcium --> 10 ppm
- 1000 mg Phosphorous --> 10 ppm
- 60 mg Vitamin C --> 0.6 ppm
- 15 mg Zinc --> 0.15 ppm
- 2 mg Manganese --> 0.02 ppm
- 80 micrograms Vitamin K --> 0.008 ppm
- 6 micrograms Vitamin B-12 --> 0.0006 ppm