Here's a short story:

A UN report said that around 60 million people across the world are drinking polluted water. Some 4,500 children die every day because of polluted water. A report showed that environmental problems kill 3 million children under five years old each year, making them one of the key contributors in more than 10 million child deaths each year. Dangerous factors include indoor and outdoor air pollution, water pollution. Another study showed that parents and scientists from seven countries including the United States and India think pollution is the biggest threat to children's living environment. Mrs Green tries to teach her daughter Susan by setting a personal example. She picks out recyclable waste and uses the water from the washing machine to wash the toilet. Chinese children mostly learn about environmental protection in school. Some non-governmental organizations and child centres also teach kids to protect the environment. "More parents have known about it. Family is now playing a more important role," says a Chinese official. Vera Lehmann, a German scientist says many Chinese now think more of pollution. "I was surprised to find many schools in China are willing to educate the children on environment," Lehmann said. "There has been a big change between now and ten years ago when I first travelled here." (End of story)

Provide your best guess for the following question. Give ONLY the guess, no other words or explanation.

For example:

Guess: <most likely guess, as short as possible; not a complete sentence, just the guess!>

Here are a few examples of questions and answers:

Question: What kind of water are millions of people drinking?
Guess: polluted

Question: How many people are drinking it?
Guess: 60 million people

Question: How many children pass daily because of it?
Guess: 4,500

Question: How many children younger than 3 die from environmental problems yearly?
Guess: 3 million children

Now answer the following question in the same format.

Question: What is one of the dangerous factors?

indoor and outdoor air pollution