I sometimes cook using a sous vide method of cooking. Like smoking, it is a low-temperature, slow-cook method. With sous vide, you place the thing to be cooked into a vacuum bag, and then put it into water that is held precisely at the temperature you want the food to be when it is fully cooked. So, if you want rare steak, which is 130 degrees F, you put the bagged food into 130 degree water, and let it sit there for several hours. It sounds awful, but it actually works really well. (You sear the steak at the end, to create the needed surface flavor).
But here's the thing: how can it be safe to eat meat that has never had any part go above 130 degrees? The same thing with chicken. I cooked a chicken breast by putting the vacuum bag into 143 degree water. The breast never got above that temperature. The USDA figures tell me that I will die if I don't get my chicken to 165 degrees.
Why am I not dead?
My reason for posting is that I found out that the simple USDA guidelines do not tell the whole story about food safety, and it isn't until you delve way down into the USDA site that you find out that you can achieve the same level of food safety at lower temperatures, but only if you keep the food at those temperatures for a long time.
If you want the hard-core science behind it, here's a link to the USDA's food safety paper on the subject:
This one is just for poultry, but there are similar papers for other food, with slightly different temperatures and times.
Here's the bottom line: the usual food safety temperatures with which we are familiar are designed to kill the bacteria almost instantly. However, those same pathogens, if exposed to lower temperatures, but for a longer time, will eventually succumb. Here is a chart from seriouseats.com which is derived from the USDA paper I linked to above:
As a result, my 143 degree chicken breast which was cooked in a 143 degree water bath was perfectly safe, but only because it was held at that temperature for more than 27.5 minutes. It was also the most amazingly moist, wonderful chicken I have ever tasted, with a wonderful texture I've never before experienced when eating chicken, without a hint of that pink, under-done poultry taste you may have experienced on Thanksgiving or Christmas with a turkey or other fowl that never quite got done.
So for most smoking that involves heat (i.e., NOT "cold smoking"), you will be OK if you use low temperatures, as long as the internal temperature of the food is at or above 135 degrees for the times shown above.
One note: you still need to make sure to get the food up to that 135-140 degree range relatively quickly in order to avoid any possible toxins created by bacteria that are allowed to breed out of control by spending too much time in the "danger zone." So, my point here is not meant to encourage anyone to let food linger for more than a couple of hours in that really low temperature zone. Instead, my reason for posting is to make sure that people don't feel they have to "nuke" their wonderful, flavorful 140 degree meat in order to get it to the much higher temperatures found on the first page of the USDA site. Also, you need to use a good thermometer, inserted into the center of the food, in order to make sure the inside of the food really is at or above 135 degrees.
So, long and slow, at any temperature above 135, will be safe, according to the USDA.