Safe to leave oven on at 180F while at work to cook ribs?

Yes, there's a risk, but it's not significantly higher than having most other home appliances turned on (eg, a lamp, dehumidifier, dish washer or dryer).

Although it heats up, a full-sized oven is insulated, and you're not operating at a very high temperature. Provided it seals well, even if there's a fire, there would be little oxygen to sustain it. I would not trust a toaster oven or desktop oven. with this sort of thing, as they're not as well insulated, nor do they tend to seal well.

As ovens are based on temperature, and will cycle on an off, rather than just feed in a constant amount of power (such as a cook top) ... so it won't get so hot that it causes other issues.

Update: I made the (possibly incorrect) assumption that this was an electric oven. I don't know that I'd trust a gas oven the same way (as I would assume that it is not well sealed) ... although people leave their home heating systems and water heaters on without shutting them off when they leave each day, so it's probably not an issue.

There are three issues here:

• The quality of the food
• The safety of the food
• The safety of your home

Food Quality

From a palatability perspective, you may or may not get a decent result, depending on how high the internal temperature of the ribs rises.

180 F is very close to the temperature that needs to be achieved to effectively convert gelatin into collagen and create the tender texture out of the tough ribs as any kind of reasonable rate. If the temperature inside the ribs doesn't rise that high--and air is a poor transmitter of heat, which is why you can stick your hand inside a 500 F oven--you may not get a great result.

Food Safety

The same issue--temperature--is key for safety.

You want the ribs above 140 F as quickly as reasonably possible to inhibit pathogen growth. It is not clear that the ribs will get that hot in such a low oven, or if they do so, if they will do it in a reasonable period of time (less than say, an hour) so you may have a potential food safety issue.

This is exacerbated by the fact that most ovens have a considerable margin of error between the set temperature and the actual temperature (which also varies above and below the set point).

Home Safety

RI Swamp Yankee has located a reference to the US Fire Administration which clearly recommends not leaving cooking appliances unattended when no one is home:

The leading cause of fires in the kitchen is unattended cooking.

• Stay in the kitchen when you are frying, grilling, or broiling food. If you leave the kitchen for even a short period of time, turn off the stove.
• If you are simmering, baking, roasting, or boiling food, check it regularly, remain in the home while food is cooking, and use a timer to remind you that you're cooking.
• Stay alert! To prevent cooking fires, you have to be alert. You won't be if you are sleepy, have been drinking alcohol, or have taken medicine that makes you drowsy.

Slow cookers are generally recognized to be safe when used unattended - and there are a variety of slow-cooker rib recipes out there. I've made them to great success before - they're fine braising in their own juices , or with a little liquid, such as root beer or Dr. Pepper.

The problem with the oven is the lack of temperature control at low temperatures and the amount of electricity it uses - with a gas oven, carbon monoxide would be a worry leaving it on for 8 or 9 hours at a stretch, as well as steep gas bills. There would be a very low risk of fire, but enough of one where fire safety experts do not recommend leaving it unattended.