Following are strictly my personal feelings, please take as such, accept or ignore.
1. MEK is a listed ingredient on several hobby cement labels. So many chemicals in the workplace, and part of many consumer products, are not something you want to have unprotected exposure to. Testors, Tamiya and Tenax list MEK, smells very strong, works well for dissolving plastic, but isn't something that will do you ANY GOOD if you are exposed to it, either in the respiratory system or through the skin.
2. Perfectly safe for use if adequate safeguards are implemented, positive airflow removing vapors from the work area, respiratory protective mask and appropriate gloves are very beneficial.
3. Extended periods of exposure I believe most certainly are capable of creating risk to your personal health, just how severe I think may be difficult to assess or communicate, but exposure is not something to guess about.
Again my personal feelings and I'm not trying to stir up a S### storm, but I feel strongly that these products that include MEK and other questionable ingredients, even some paints, should be used with appropriate caution.
Masks, airflow and gloves can go a very long way, toward helping you preserve your health and wellness. You only get one set of lungs, best to protect them the best that you can.
I wish for my fellow modelers to stay safe and be well.
Patrick