If an astronaut’s helmet was to accidentally come loose or come off completely in space, then obviously, this would be very bad news indeed.

First of all, if a warm body is exposed to a vacuum or near vacuum it will not implode or explode. When astronaut takes off his helmet, the following things will happen.

His head would be exposed to the temperatures of space, which is -455 degrees Fahrenheit.

The air in your lungs is FORCED out of your body due to the surroundings being a vacuum.

You will have 15 seconds before the oxygen in your bloodstream runs dry. Your circulatory system doesn’t know that there is a lack of oxygen, so it cycles blood throughout your body even if there is no oxygen. That really messes with your system, especially the brain.

The radiation from the sun would hit you with crazy intensity that would harm you even if you somehow managed to make it out alive.

As the helmet was removed all the suit air would rush out. The lungs would collapse as well if the mouth was open. If not, the lung would explode as the air inside them attempted to get out into the near vacuum. If you could get the astronaut back into pressure within about 3–4 minutes there is a good chance he would eventually recover with minimal after effects.