Does it hurt a 16 year old to be given nitrous oxide from a dentist for 2 hours without being monitered.?

Probably not: This is generally "safe", but everything has some risk. In a reputable dentist office with competent staff, this should not be a problem to receive nitrous oxide. But every patient is different, and if you are concerned, have medical issues, or have a history of anesthetic complications, you can request consultation with an anesthesiologist if necessary (but it will be extra expense).

It may: If given without oxygen delivery and oxygen and co2 monitoring the potential for harm is much greater and i would not allow it , esp if given with a sedative. If the patient has a eustacian tube problem it may rupture the eardrum and or cause nausea.

Yes: Nitrous is very safe, and in low concentrations especially so. But it is standard of care to monitor with at least pulse oximetry. If something goes wrong, and higher concentrations of nitrous occur, low blood oxygen levels could result, and if missed can cause brain damage, or death.

Hurt no? Bad...yes!: Nobody should be unmonitored and getting nitrous oxide. At the least oxygen saturation monitoring needs to take place.

It depends: How much oxygen was also added? The patient will get cyanotic unless is colored or anemic which is a form of monitoring. But this day and age you must be crazy to take your family to someone that does not monitor and gives a hypoxic mixture. Hypoxia can cause brain damage!