Another boost came from people flying on low-cost, domestic airlines such as Las Vegas-based Allegiant Air, which has grown to become the sixth-largest carrier at McCarran and is close to displacing American in the top five.

In addition to an abundance of low-cost airlines serving Las Vegas, Southern Nevada's population continues to boom, which means more local people using the airport. That leaves Clark County Department of Aviation workers to figure out how best to accommodate traffic that is stretching the limits of what McCarran was designed to handle.

"We have to continue to look at ways to push more people through a facility that wasn't built to accommodate this many people," airport spokesman Chris Jones said.

The aviation department is in the midst of a $4 billion capital improvement project aimed at maximizing capacity at McCarran, a plan it hopes can keep the airport running smoothly until another major airport is up and running about 25 miles southwest of the current facility.

But the new airport proposed for the Ivanpah Valley faces opposition from conservationists who don't like the proximity of the proposed site to the Mojave National Preserve in California. The earliest the new airport could operate is 2017.

In the meantime airport officials are spending $1.8 billion for a new terminal at McCarran that is scheduled to open in 2011. They are also expanding existing baggage claim carousels and security gates, among other projects.

Although the seemingly frenetic pace at McCarran fuels passenger complaints about everything from taxi lines to security backlogs the airport ranks among the highest in America in third-party customer service surveys. In 2006 J.D. Power and Associates ranked McCarran No. 1 among major North American airports. It ranked fourth in 2007.

The airport was designed to handle about 42 million passengers annually but accommodated 43 million in 2006. Airport officials have added off-site baggage checking, off-site rental car service and common use ticketing kiosks to keep passengers moving.

"This growth is coming without any apparent driver," Jones said of the July passenger figures. "It just is illustrating the challenge that we face."

Southwest remained the carrier with the most traffic at McCarran, moving nearly 1.5 million people through Southern Nevada during the month, an increase of more than 7 percent.

US Airways, United, Delta and American rounded out the top five. Although Delta was the only one of those carriers to post an increase for the month. Delta logged 224,000 passengers, up 7 percent from July 2006.

Allegiant, a relatively new carrier that specializes in routes between Las Vegas and small towns with little air service, surged into the sixth spot with a month-over-month increase of 33 percent to almost 174,000 passengers. That puts it just behind American which had its July passenger count decline almost 9 percent to a little more than 178,000.

Allegiant spokeswoman Tyri Squyres attributed the airline's popularity to low fares and an effort to create its own market share, as opposed to fighting to acquire customers from other airlines.

Allegiant regularly adds new, direct routes to Las Vegas from small towns like Duluth, Minn., and Kileen, Texas. The airline's average one-way fare was just $87 during the last quarter and it often packages tickets with hotel rooms. The goal is to convince small town people to choose a trip to Las Vegas over other options for discretionary spending like shopping.

"When we go into a new market we stimulate the market and increase demand," Squyres said. "People will take a trip they hadn't planned on taking or they will take additional trips."

She cited a 93-year-old woman who was on Allegiant's inaugural flight from Duluth to Las Vegas in 2006 as an example. The woman had never flown nor been to Las Vegas before the Allegiant service.

"It was her chance to go to Vegas," Squyres said.