I would just list some most confusing points in using these two words.
When external and exterior refer to the outside of something, say 'The exterior walls need a new coat of paint', apart from exterior being able to be used as noun, are these two used interchangeably?
When they express something coming from outside, like 'Low birth weight may be caused by external factors', 'information from external sources' external affairs' and 'external examination', can I replace external in these sentences by exterior?
Best Answer
External can often be substitued for exterior. But not the other way around. Let's look at the definitions:
Exterior usually refers to the outside surface of an object. In your example "exterior walls" are part of the house, but are facing outwards. Merriam-Webster defines exterior as
As you can see in the definition, external can also mean exterior surface, for example you could call them "external walls", but this would be a less common usage.
In practice, external usually refers to a force or object outside of the subject. In your example "external forces" are forces from the environment outside of the control or influence of the infant, but not part of the infant themselves.