If you have someone who can't use a whistle, such as someone who is breathing-impaired, you might forego the whistle and get them a personal alarm. This is what I got for my Mom, who had a heart/breathing problem. It is 120 dB, is activated simply by breaking the connection between the alarm and the strap, and the alarm will sound for 20 minutes. You can also use it to make a three-whoop+pause signal fairly easily.

But my Mom said if it was activated for even a second, it would blow out her hearing aid batteries.

Sue