Analysis of air bubbles trapped in fossils, such as in fossil amber, show that our air in earliest times contained about 35% oxygen. Today, the average oxygen content of our air is approximately 20%. In some larger, more polluted cities, oxygen content in the air has been measured at 12 - 15%.
There are 297 words which start with 'pen'. 3 letter words starting with 'pen': pen. 4 letter words starting with 'pen': pend. pens.
A pen name, or nom de plume (French for pen name), is a pseudonym (sometimes a particular form of the real name) adopted by an author (or on the author's behalf by their publishers). Some female authors used male pen names, in particular in the 19th century, when writing was a male-dominated profession.