Why does the speaker join the twenty-nine pilgrims at the inn?

The narrator joins the pilgrims so that the story becomes more intimate, personal, and realistic.

Although you have not provided the text that your question refers to, we can see from questions similar to yours that you are referring to "The Canterbury Tales" written by Geoffrey Chaucer.

In "The Canterbury Tales" we are introduced to:

Geoffrey Chaucer positions himself as the narrator and character of this story, as he shows that he sits together with the pilgrims to hear what they have to say.

He does this to create a more intimate atmosphere, showing the story as an experience he had.

But he also does this to make the story more realistic, as it shows that he is telling about something he participated in and witnessed.

You can find more information at the link below:

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The narrator in  Geoffrey Chaucer's "THE CANTERBURY TALES" joins twenty-eight pilgrims in order to make the account of the incident look more real.

Explanation:

Geoffrey Chaucer is considered The Father English Poetry and similarly he is first realist of English literature. By making the narrator join the twenty-eight pilgrims at the inn, Chaucer make sure that his poetry be considered realistic. The narrator himself becomes a character who is not free of biases and his own prejudices.

i guess the cause and effect would show how being around the smoke(the cause) can have negative effects on the body.

explanation: