While the saying “Whatever doesn’t kill you makes you stronger” may be popular among those who skirt tragedy through life, it is not an attitude we can adopt as engineers.

As professionals, we should endeavor “to protect the environment and the safety, health, welfare and property of the public.” This is the No. 1 principle of ASPE’s voluntary code of ethics. When it comes to Legionella, there are new standards and methods being developed to help us protect the public. We should become familiar with these, and understand how Legionella affects the public practically.

If you ever wanted to be overcome by long strings of multisyllabic words, try reading a scientific report on Legionella. I learned that infection occurs when the bacteria is placed in juxtaposition with alveolar macrophages. How about this: Legionella is an “aerobic, pleomorphic, flagellated, non-spore-forming Gram-negative bacterium.” If we memorize a few of these lines we should really be able to impress some folks.

Engineers are at their best when they find practical solutions to issues that concern the public. In this case, it’s to make sure that Larry and Lola Legionella don’t start a party and drift into somebody’s lungs. One of the questions I’ve been wondering is: What size is a droplet of water that would carry a colony of these critters deep into somebody’s pulmonary bagpipes?

If your daily engineering routine is anything like mine, you’re probably educated on technologies and techniques to help design systems that are safer for the public. When we design these systems, we have an opportunity to be creative, work with manufacturers who develop new technology, talk to facility staff who maintain systems and figure out what owners need.

When we design systems that prevent Legionella, we should be looking at all sections of the system including incoming water, hot water generation, recirculation, areas of flow stagnation and water dispersion at the point of use. We should be designing systems that are clever and informative. As we become more accountable for our designs, why not include elements that provide information that reports how the system is performing.

Much of the work we do involves renovations to existing facilities. Considering the challenge of eliminating Legionella throughout one of these buildings, what can we do to prevent the bacteria from becoming airborne? For me, visualizing tiny vessels of water droplets is important to understanding where they could come from, whether it is an aerator on a faucet, nebulizer or cooling tower.

In my March column, we looked at the average weight of a 1-mm raindrop and found there to be about 125 raindrops in a grain. You may recall that there are 7,000 grains in a pound of water. I can’t remember the last time I inhaled a raindrop, so I figure a Legionella-laden droplet to be much smaller, on the order of a mist. According to the NIH report referenced previously, as well as other standards, 5 micrometers seems to be the size droplet that can be inhaled “deeply into the lungs” (ASHRAE 12 § 4.1.3).

If the size of a water droplet that can be inhaled is .005 mm in diameter, and an average raindrop is 1.0 mm in diameter, that means there are about 8 million droplets of mist in one raindrop.


0.52 (mm^3)/raindrop divided by 6.5 x 〖10〗^(-8)  (mm^3)/(mist droplet) = 8,000,000 droplets

of mist in one raindrop.


Taking a look at the weight of these mist drops, we can see that there are about 1 billion of them in a grain, which translates to 7 billion in 1 pound of water.


125 raindrops/grain x 8 million

mistdrops/raindrop = 1 billion mistdrops per grain.


It is our professional responsibility to design systems that protect the public safety, especially those who are the most susceptible to infection and disease. While we embrace new technology that can help mitigate the growth of Legionella, it is important to remember how the Legionella is transmitted through the air and take the “mistery” out of how it can be a threat.


A note about my March column, “Lucky’s Rainbow”: A reader found a mistake in the formula where I calculated the weight of water that could pond on a supermarket roof with a 3-foot parapet. The correct unit conversion would include 8.33 pounds per gallon and 7.48 gallons per cubic foot. This would translate to 7.5 million pounds of water, not 1 million. Thank-you for pointing this out.