I don't think it'll be a surprise to anyone that coffee is bad for your teeth. You've likely been lectured by your dentist about it for years, along with other dark liquids like red wine and soda. But even more surprising? It's likely that tea is worse for your teeth than coffee, despite the color of it usually seeming at least a little lighter. However, tea—especially black tea like Earl Grey or English Breakfast blends—contain high levels of chromogens, tannins, and acids, all of which make tea bad for your teeth. contribute to staining teeth and eroding enamel, the hard, thin, transparent coating that protects your teeth. While coffee contains these things, too, it has a much lower tannin content.
So, let's start with teeth, and why they're so prone to staining in the first place. Enamel is the hardest substance in the body, but while it looks smooth, actually isn't: There are nicks and cracks on the surface that just get worse as you get older. Foods and drinks that stain can make their way into these cracks and cause some harm.
As you know from science class, over time, habitual exposure to acids cause many substances to break down, including tooth enamel. Acids soften the enamel, and make it more susceptible to damage, exposing the tooth underneath. Tannins are astringent, plant-based compounds found in drinks like wine, too. Because of their ability to bond to other substances, they have a bad habit of making other substances stain. When that substance is high in chromogens—substances that, when oxidized, become deeply colored—you've got a perfect recipe for discoloration. Because tea contains a higher amount of tannins, the staining can be that much more powerful.
If you're on a quest for whiter teeth, there's, fortunately, a pretty easy thing you can do: brush your teeth—preferably with a whitening toothpaste—right after you finish gulping down your tea. If bringing a toothbrush to the office seems unlikely to happen for you (same), you can at least rinse out your mouth with water. It's better than nothing and does an alright job of removing staining substances.
Of course, you could also give up tea. But that sounds like a horrible idea.