**Particles settle out of the air at different rates depending on the size of the particle. These tests show the relative exposure from particles at different distances from the source.**

To determine what happens to aerosol particles at various distances we did some tests using an atomizer and 4 particle counters. We created an imitation “sneeze” at two distances – 6 feet and 10 feet – from the particle counters and recorded the particle counts at 0.5um, 1um, 2.5um and 5um. The chart above shows the results.

Many people assume that the potential exposure to particles from a sneeze is proportional to the distance from the source. Since the difference in the distance between 6 feet and 10 feet is 4 feet. By this assumption, the particles reaching the particle counters should be about 66% greater from the 6 foot source versus the 10 foot source. Actually, particles radiate out from the source in all directions. Therefore, the exposure is actually the cube of the distance. In this case 6 feet to the third power is 216 while 10 feet to the third power is 1000. Therefore, we would conclude that the exposure particles would be somewhere around 5 times greater from the 6 foot source than the 10 foot source. This is why doctors have said for years that the greatest danger of contracting a disease (like the flu) from an infected person is either by touching a contaminated surface and not washing hands sufficiently or by being relatively close to the person with the disease.

Our graph is a bit difficult to read so let me summarize some of the results in this narrative. The horizontal axis is minutes from the initial shot with the aerosol. The verticle axis is the particle count per cubic foot in 100’s. So at 5 minutes the particle count at 0.5um from 6 feet was 255,000 and from 10 feet it was 80,000. At 1um the particle count from 6 feet was 63,000 while the particle count from 10 feet was 18,000. At 2.5um the particle count from 6 feet was 5,400 while the particle count at 10 feet was 14,500. At 5um the particle count from 6 feet was 2,100 and from 10 feet it was 500. At 10 minutes the particle counts were the following: for 0.5um – 6 feet=164,100 and for 10 feet=88,900; for 1um – 6 feet=45,400 and for 10 feet 18,100; for 2.5um – 6 feet=30,800 and for 10 feet=18,100 and at 5um – 6 feet=900 and for 10 feet=300. The particle counts remained about the same for the rest of the 15 minutes in the test.

The numbers in our tests are relatively close to the projected results. Showing that the closer the source, the greater the exposure by a far greater amount than just the actual distance from the source.

Another point to consider is that particles are three dimensional. That means that a particle that is 5um in diameter is not 10 times greater than a particle 0.5um in diameter – it is actually 1,000 times greater. Therefore, it would contain 1,000 times the level of viruses or bacteria. To put this in perspective, in our tests if we take the number of particles at 0.5um after 5 minutes and assume that this is a baseline, our exposure to infection from this size particle would be 255,000 while the exposure to infection from the 5um particles would be at least 2,100,000 – 9 times greater.

Larger particles settle out of the air quicker than smaller particles. For example, a 5um size particle will settle at a rate of 2.5 inches per minute while a 0.5um size particle will drop out of the air at a rate of 1.4 inches per hour. This is why we see such a significant drop in 5um particles in our tests. The particles that settle out of the air will fall on and contaminate surfaces in the space. So not only can one be exposed to infectious agents by touching a spot where an infected person has touched, one can also be exposed to the infectious agents from a sneeze our cough from an infected person that have settled on surfaces.

Therefore, while it is possible that one could contract an infectious disease through airborne transmission of small infected particles from the cough or sneeze of an infected person, it is much more likely that the vast majority of diseases are transmitted within an indoor envirornment either by direct contact with the infected person, touching an infected surface or by being in close proximity to the infected person.