Longest English Word

Most of the children remember a 28 - 45 lettered word as the longest English word.
Some of these words are:

• antidisestablishmentarianism-opposition to the disestablishment of the Church of England - 28 letters
• floccinaucinihilipilification -the estimation of something as worthless - 29 letters

But Medical students know a word that is even longer than these i.e.

• pneumonoultramicroscopicsilicovolcanoconiosis-a supposed lung disease - 45 letters

Surprising fact is that even this word with 45 letters is not the longest. But there are some chemical compounds. These Compounds might have infinite letters, like a string of DNA structure.
One of them is a  Tobacco Mosaic Virus, Dahlemense Strain. .containing 1185 words.

(1,185) ACETYL­SERYL­TYROSYL­SERYL­ISO­LEUCYL­THREONYL­SERYL­PROLYL­SERYL­GLUTAMINYL­PHENYL­ALANYL­VALYL­PHENYL­ALANYL­LEUCYL­SERYL­SERYL­VALYL­TRYPTOPHYL­ALANYL­ASPARTYL­PROLYL­ISOLEUCYL­GLUTAMYL­LEUCYL­LEUCYL­ASPARAGINYL­VALYL­CYSTEINYL­THREONYL­SERYL­SERYL­LEUCYL­GLYCYL­ASPARAGINYL­GLUTAMINYL­PHENYL­ALANYL­GLUTAMINYL­THREONYL­GLUTAMINYL­GLUTAMINYL­ALANYL­ARGINYL­THREONYL­THREONYL­GLUTAMINYL­VALYL­GLUTAMINYL­GLUTAMINYL­PHENYL­ALANYL­SERYL­GLUTAMINYL­VALYL­TRYPTOPHYL­LYSYL­PROLYL­PHENYL­ALANYL­PROLYL­GLUTAMINYL­SERYL­THREONYL­VALYL­ARGINYL­PHENYL­ALANYL­PROLYL­GLYCYL­ASPARTYL­VALYL­TYROSYL­LYSYL­VALYL­TYROSYL­ARGINYL­TYROSYL­ASPARAGINYL­ALANYL­VALYL­LEUCYL­ASPARTYL­PROLYL­LEUCYL­ISOLEUCYL­THREONYL­ALANYL­LEUCYL­LEUCYL­GLYCYL­THREONYL­PHENYL­ALANYL­ASPARTYL­THREONYL­ARGINYL­ASPARAGINYL­ARGINYL­ISOLEUCYL­ISOLEUCYL­GLUTAMYL­VALYL­GLUTAMYL­ASPARAGINYL­GLUTAMINYL­GLUTAMINYL­SERYL­PROLYL­THREONYL­THREONYL­ALANYL­GLUTAMYL­THREONYL­LEUCYL­ASPARTYL­ALANYL­THREONYL­ARGINYL­ARGINYL­VALYL­ASPARTYL­ASPARTYL­ALANYL­THREONYL­VALYL­ALANYL­ISOLEUCYL­ARGINYL­SERYL­ALANYL­ASPARAGINYL­ISOLEUCYL­ASPARAGINYL­LEUCYL­VALYL­ASPARAGINYL­GLUTAMYL­LEUCYL­VALYL­ARGINYL­GLYCYL­THREONYL­GLYCYL­LEUCYL­TYROSYL­ASPARAGINYL­GLUTAMINYL­ASPARAGINYL­THREONYL­PHENYL­ALANYL­GLUTAMYL­SERYL­METHIONYL­SERYL­GLYCYL­LEUCYL­VALYL­TRYPTOPHYL­THREONYL­SERYL­ALANYL­PROLYL­ALANYL­SERINE

Some other examples are:

Word	Letters	Characteristics	Dispute
Methionylthreonylthreonylglutaminylarginyl...isoleucine	189,819	Chemical name of titin, the largest known protein	Technical; not in dictionary; disputed whether it is a word
Methionylglutaminylarginyltyrosylglutamyl...serine	1,909	Longest published word[1]	Technical
Lopadotemachoselachogaleokranioleipsano...pterygon	183	Longest word coined by a major author,[2] the longest word ever to appear in literature.[3]	Coined; not in dictionary;Ancient Greektransliteration

But We don't tend to regard these terms as proper 'words'.

These might  be the names of chemicals and can be treated as terms, but these are not english language words.

Here we have another name :

Name of a place containing 85 words.

TAUMATA­WHAKA­TANGI­HANGA­KOAUAU­O­TAMATEA­TURIPUKAKA­PIKI­MAUNGA­HORO­NUKU­POKAI­WHENUA­KITANA­TAHU
A hill in New Zealand. This Maori name was in general use, but is now generally abbreviated to Taumata. The name means: the summit of the hill, where Tamatea, who is known as the land eater, slid down, climbed up and swallowed mountains, played on his nose flute to his loved one.

But this and some other more like this  can't be found in major dictionaries. 

Conclusion: 

• Pneumonoultramicroscopicsilicovolcanoconiosis is the Longest word in a major dictionary. But it is technical as well as coined.
• Floccinaucinihilipilification is the Longest unchallenged nontechnical word. But it is also coined. 
•  Antidisestablishmentarianism is the Longest non-coined and nontechnical word, having 28 letters.

References:

http://en.wikipedia.org/wiki/Longest_word_in_English

http://www.oxforddictionaries.com/words/what-is-the-longest-english-word

http://www.fun-with-words.com/word_longest.html