In the midst of the nation's worst soot problem this season, air authorities are issuing stronger warnings when pollution is at a much lower level than before -- a new protective measure.

The San Joaquin Valley has been caught in a seven-week dry spell that shows no sign of relief. The lack of cleansing storms has allowed a haze of tiny debris to build up and violate the federal health standard for 37 consecutive days.

During that time, hourly readings in Fresno and Clovis have spiked nearly four times higher than the daily health standard. Yet stronger alerts weren't issued because an old threshold for warnings of very unhealthy air was not crossed.

Now the San Joaquin Valley Air Pollution Control District will warn people when pollution spikes are twice the daily health standard. That was not nearly enough to trigger a similar warning in the past.

School administrators, teachers, coaches, parents and anybody else can sign up on the district's website to get email notifications about changes in air quality on the hourly warning system.

The warning system, which the district started in 2010, is the only one of its kind in the country. Participation in the system is voluntary. The district has no authority to force anyone to stop outdoor activities when pollution spikes.

But officials say schools and the rest of the public should heed the warnings.

The air district worked for many months adjusting the online, hourly warning system and launched the revisions just after Christmas.

They say the only hitch now is that people may be confused by their changes. The confusion centers on two separate air rankings online -- the district's daily air forecast and the hourly warnings. Until now, both have used a five-color system to rank the air quality in a range between healthy (green) and very unhealthy (purple).

Now the hourly warnings will show air quality in five numbered levels, the fifth being very unhealthy. The idea is to make a clear distinction between hourly spikes and all-day averages.

Kevin Hall, executive director of the Central Valley Air Quality Coalition, says the district should have kept it simple to avoid confusion.

"There's been a lot of money invested in the color system over the years," he said. "We should stick with it."

Behind the colors-versus-numbers discussion, there is a science-based reason for the change. Under the 1997 standard, pollution spikes would have to rise above a lung-searing 150 micrograms per cubic meter before schools would be informed that outdoor activities should be canceled. Now, the threshold is 75 micrograms per cubic meter -- still a hefty reading.

For perspective, a federal health violation occurs when an air monitor averages more than 35 micrograms for the day. Since Dec. 3, there have been many hourly spikes well beyond 100 micrograms.

"You shouldn't be outside exercising when particulates reach these levels," said David Lighthall, district science adviser.