Do not use symbols related to religion or culture. They might be misunderstood or even give offense. For example, in some cultures birds symbolize bad luck, and in other cultures a skull and crossbones represents danger.

You should also consider the significance of certain numbers. For example, in some countries the number 13 is considered unlucky. Buildings often do not have a floor 13. The number 86 is often used to imply removing or stopping something as in "86 it." In parts of Asia the number 4 is pronounced the same as the word for death and is considered morbid. Not all numbers have a negative connotation. Some people consider the number 7 to be lucky.

Unless you have done an extensive study of numbers and their meanings in many cultures, you should avoid this type of reference to numbers or symbols.