Final answer:
The dose equivalent limit for a radiography student under the age of 18 is not specified in the information provided, but diagnostic radiation doses are regulated to be as low as reasonably achievable to protect health, such as 0.1 mSv for chest X-rays and about 5 mSv for dental X-rays.
Step-by-step explanation:
The dose equivalent limit for a radiography student under the age of 18 is not explicitly stated in the provided text. However, it is important to note that medical doses of radiation are limited and diagnostic doses such as those from X-rays are generally low. These measures are especially important for minors in training or educational settings, as their long-term health must be protected. For example, a chest X-ray gives a dose of about 0.1 mSv to the affected tissue, while dental X-rays can give about 5 mSv (0.5 rem) per x-ray. The regulations and limits for radiation exposure can vary by country and are set by respective health and safety authorities like the U.S. Nuclear Regulatory Commission (NRC) or equivalent bodies in different countries.