This word, which refers to a lung disease caused by inhaling very fine silica dust, has 45 letters.
However, there are some technicalities:
* It's not officially recognized by all dictionaries. Some dictionaries don't include it, or consider it a made-up word.
* There are longer words in specialized dictionaries. For example, the *Oxford English Dictionary* includes the word floccinaucinihilipilification, which has 29 letters and refers to the act of estimating something as worthless.
* There's a debate about the definition of "word." Some argue that long, technical terms are not true words, but rather combinations of existing words.
Ultimately, the answer depends on what you consider to be a "word" and which dictionary you're using. But pneumonoultramicroscopicsilicovolcanoconiosis is certainly the longest word found in most general dictionaries.