Usage History of 'tetrapyrrole'

The following chart shows how frequently 'tetrapyrrole' appeared in printed materials over time. For example, the word 'tetrapyrrole' occurred 6 times in books in 1937 and 1,035 times in 2008. From 1937 to 2008, the word 'tetrapyrrole' was used a total of 24,885 times in books scanned by Google. See Reference: Google Ngram project.

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