Which of the following frequencies has the lowest Maximum Permissible Exposure limit?

The correct choice relates to the concept of radio frequency (RF) exposure limits, which are established to ensure safety when dealing with electromagnetic radiation. The Maximum Permissible Exposure (MPE) limits vary with frequency, with lower frequencies generally having higher MPE values.

At lower frequencies, such as 3.5 MHz, the body absorbs energy differently compared to higher frequencies. The biological effects of RF radiation are influenced by the frequency; thus, higher frequencies tend to have stricter exposure limits due to more significant biological interactions, especially concerning heating effects on tissues.

In the context of the question, 50 MHz, although higher than 3.5 MHz, is still relatively low in the spectrum. Meanwhile, the frequencies at 440 MHz and 1296 MHz are much higher and would typically be associated with lower MPE limits due to their increased potential for absorption and heating effect in biological tissues.

Overall, the 50 MHz frequency stands as having a lower Maximum Permissible Exposure limit when compared to the other frequencies listed due to the different ways the human body interacts with RF energy at these respective frequencies.