Shipping containers, by design, are wind and water tight; so the automatic conclusion that many people make is "how do I get fresh air when I'm locked inside of my shipping container home/storage unit?" The answer is simple, once you add doors and windows, and probably an HVAC system, there will be more than enough airflow to allow you and your family to survive. Even if you're using it as a storage unit and don't add doors or windows there are small breather vents on either end of the container that provide minimal airflow.
Since we've established that running out of air isn't an issue inside of a shipping container house or storage unit, there are other potential issues due to the lack of freshly circulated air: mold, rust, and other diseases.
Mold is formed in dark places when trapped air condensates and allows fungi to grow. Once mold takes hold of something it can be expensive, if not impossible, to fix. Mold is less of a concern in a container house, but when left unmanaged inside of a storage container it can be bad news for the stored contents.
Riding on the back of mold is disease. If mold is inhaled it can cause all types of nasty health problems. Sometimes there's a "damp" smell with mold, other times you can breathe it in and not even know it. To stay healthy when removing mold it's important to wear a face mask.
In areas where there is a large shift in day/night time temperatures condensation can form inside of a shipping container. If condensation is trapped the moisture will begin to lay and can expedite the process of rust and corrosion.
If you're building a shipping container home, ventilation shouldn't be an issue because the heating and air conditioning system should provide this function. On top of this, opening and closing doors and windows on a regular basis will unintentionally keep the air flowing.
Ventilation in a storage container can be a little trickier, especially if you don't access the unit on a regular basis. The simplest solution is just to install vents of some type. The two most common are the whirly bird vents that sit on the top of the container and blow in the wind, or larger container vents that can be welded to the sides of the container. Both types have their pro's and con's, it's really dependent on the area and climate where the container will be.
ContainerLockBox.com is a subsidiary of ContainerAuction.com, and provides after-market parts for shipping containers. All shelving, bolt on lock boxes, and shipping container vents are manufactured in the United States.