1.   Among those most sensitive to fine particle pollution are infants and children, the elderly and individuals with pre-existing heart and lung conditions.

2.   California regulators are in the process of beefing up standards for particle air pollution, but not of the size that worries Ritz and her UCLA colleagues.

3.   Fine particle pollution comes from a variety of sources, the largest of which is coal-fired power plants, the report said.

4.   In the West, more of the particle pollution comes from vehicles and consists of nitrogen-based compounds.

5.   Other researchers said that poor summer visibility in the form of white or brown haze is an indication of high levels of particle pollution.

6.   Sources of fine particle pollution include diesels, cars, coal-fired power plants and industrial boilers.

7.   The deaths associated with particle pollution are mostly among people who already have breathing problems, including asthmatic children and some elderly people.

8.   The natural-gas buses also produced too little particle pollution to be detected, though they produced more carbon dioxide, a significant greenhouse gas.

9.   The U.S. Environmental Protection Agency is considering revisions of its rules for particle pollution.