What is the risk of contracting Legionnaires’ disease from a wood cookstove?

I have an old-fashioned wood cookstove (cast iron) in my log cabin which I visit on weekends. There is a “water jacket” on one side, a tank that you put water in to humidify the air (and, in the old days, to heat water for washing clothes, etc.) The lid has a few holes in it, or you can open the whole lid. When the fire is hot, the water gets quite warm, and steams, but doesn’t boil in the tank. This weekend we made the first fire of the season. When I lifted the lid on the water tank, I saw to my dismay that some of the water had not been emptied in the spring. There was about an inch of standing water in the bottom. There is no spigot, unfortunately (which is why it’s easy to forget to empty it), so I began to ladle it out into a bucket. Got most of it, and then started to get nervous about bacteria / Legionella and so I poured a large pot of boiling water into the tank to hopefully kill everything in the remaining water. Let it sit for a little while, lid closed, and then when it had cooled a bit, ladled it all out and cleaned the tank as best I could with vinegar (tank is now dry). The water was still steaming when I was ladling it out. I wore a mask while ladling it out, but there was steam in the air and I didn’t wear a mask before or after removing the water. Also, the woodstove is in a dark corner of the cabin, nowhere near the sun. Even in summer, it doesn’t get too hot inside. Maybe 80 degrees Fahrenheit tops.

1. What is the risk if my husband or I inhaled some of the steam from the mix of old and boiled water? We are both very healthy non smokers in our mid 50s. And we obviously didn’t drink it!

Risk of exposure would be low given the description you have provided.

2. In general, should we use this water tank for humidity? The water often sits in the tank for many days while a fire is going, or if we leave before the fire is totally out. Is this unsafe?

Since you have access to the water, you could check the temperature. If above 140F/60 C Legionella would not survive.