If using a keep cup for your daily coffee fix is your way of keeping the planet healthy, make sure to clean it regularly.
Rotorua GP Dr Cate Mills told First Up said dirty cups become a breeding ground for viruses and bacteria.
Auckland cafe owner Nigel Cotter said his baristas were regularly handed reusable cups with yesterday's leftover coffee still in it.
"One of them was just full of like just drink and mould and crap, it was terrible. I don't know how long it had been sitting in his car or on his shelf, but it was weeks and it was terrible. We just said, 'hey, this is not okay' and so he was like, 'oh, sorry'."
Cotter said the staff at his cafe, Crave, wash all keep cups before pouring coffee into them.
"We will always kind of rinse it with the hot water tap just to heat the cup up anyway, and that gives you know, like 100 degree water into it.
"A lot of our customers though, who are regular, who bring in not a terribly filthy cup, but just a dirty cup."
Do you use "keep cups" for your daily coffee from barista?
Dr Mills said people needed to be aware of the health risks of using a dirty cup.
"There's potential that has infection sitting inside it. It could have bacteria, viruses sitting inside it from not being washed."
And she recommended it was best not to let a cup get mouldy.
"They'll either ingest that mould so that mould could make them feel a little bit unwell. It's unlikely to make them sick, but for someone who is immunocompromised, for example, it could make them sick and someone who was allergic to moulds, it could make them feel unwell."
Cotter said he has many regular customers who use a keep cup.
"We offer a discount for a keep cup so you get 30 cents off if you have that. And our theory is it cost us 30 cents for a cup and a lid, so yeah, we'll say, you know, they save that money and so we'll give them that discount."
He said most baristas understood people could forget to wash their cups in a hurry, so it was best to acknowledge it if you handed over a dirty cup.
"If they come in and they're like, 'oh, sorry, I haven't had a chance to [wash it], it was in the car', and it's like, 'no problem at all'. But if people come in and then don't say anything, just give you a terribly dirty cup, that's rude and disrespectful. I think, you know, to our baristas."
But he said there was another hidden cost to keep cups.
"There is a time penalty to us though, so it doesn't cost the same as a takeaway cup because there's a rhythm you get into with normal takeaways and you don't get to have that rhythm with a keep cup.
"I sort of sit with that real tension of 'we wanna be good for the environment' but the business needs to operate as a business. And sometimes those things just need a little balance. So some keep cups is good. If they're all keep cups, it would be a disaster."
During their busiest times, the baristas at Crave made up to 100 cups of coffee in one hour - having too many keep cups slowed down the production line drastically.
And rinsing those cups slowed things down even further.
"Everyone who comes into Crave, they want their coffee fast, they want their coffee in four minutes, but if the baristas have to stop, and relook and figure out which cup is which cup and are they next to the dockets? There's just a process that you've got to go through and it makes life a bit more difficult."