The air in Estes Park and RMNP was clean when I arrived in 1978. In recent years, air particulates and smog have become routine. Photography provided me a first clue - because mountain vistas that had been strikingly clear on most days are now dulled by pollution-related haze. Medical research published in peer-reviewed journals makes clear that particulates and smog cause injury to lungs. News articles often announce advisories against strenuous exercise in national parks, wherein pollution can exceed that of nearby urban areas. Irony: hiking in RMNP and living along the Front Range may be hazardous to your health. Consider two news items. "USC study links smoggy air to lung damage in children" and "U.S. smog limit permits subtle lung damage".

An overview in a news article:

Kids with asthma head indoors during smog season
http://www.ajc.com/news/atlanta/kids-with-asthma-head-568082.html

Peer-reviewed studies free online:

Diesel Exhaust and Asthma: Hypotheses and Molecular Mechanisms of Action
Environmental Health Perspectives Supplements Volume 110, #S1, Feb 2002
http://www.ehponline.org/members/2002/suppl-1/103-112pandya/pandya-full.html

Health effects of real-world exposure to diesel exhaust in persons with asthma
Zhang JJ et al. Res Rep Health Eff Inst. 2009 Feb;(138):5-109.
http://pubs.healtheffects.org/getfile.php?u=428

"In conclusion, short-term exposure to urban roadside diesel traffic led to consistent and significant reductions in lung function, accompanied by airway acidification and neutrophilic inflammation. Our findings help to explain the epidemiologic evidence on diesel traffic health effects in persons with asthma."

An article about NY city smog offers a succinct summary of adverse effects of smog:

Breathing too much ozone can trigger or worsen a wide range of serious health problems in the short term, including asthma emergencies, bronchitis and emphysema. For some people, inhaling high levels of ozone can lead to chest pain, wheezing, throat irritation, and congestion. Over the longer term, repeatedly breathing the levels of smog we have in New York City can reduce lung function and permanently scar lung tissue. Children, the elderly, people with lung disease and people who work or exercise outdoors are all at greater risk than the general population.
http://www.gothamgazette.com/article/Environment/20100804/7/3328

See also:

Bicycle riding in polluted air
http://www.timesonline.co.uk/tol/news/environment/article7140213.ece

Particulate Matter Air Pollution and Cardiovascular Disease: An Update to the Scientific Statement From the American Heart Association.
http://www.ncbi.nlm.nih.gov/pubmed/20458016

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