The White House is weighing military options in response to a chemical weapons attack allegedly carried out by the regime of Syrian President Bashar al-Assad in a rebel-held suburb of Damascus. United Nations inspectors are in Syria gathering evidence of the attack, which happened last week and left hundreds dead, while a separate U.S. intelligence report on the incident is expected later this week.

President Barack Obama declared a year ago that the use of chemical weapons by the Assad regime would cross a "red line" and could provoke U.S. military intervention. [Related: "Word in the News: Red Line."]

But what makes chemical weapons attacks more unacceptable than conventional military attacks, which have been raging in Syria since the start of the civil war there more than two years ago? And why is identifying the precise nature of last week's attack, including the chemical agents used, so difficult?

We spoke to chemical weapons expert Alexander Garza to get some answers.

A doctor and former Assistant Secretary for Health Affairs and Chief Medical Officer for the U.S. Department of Homeland Security, Garza is associate dean and professor of epidemiology at the Saint Louis University College of Public Health and Social Justice.

In a war that has seen 100,000 deaths, what is it about chemical weapons that crosses a "red line"?

The reason is that most of the world has come to an agreement that chemical weapons should not be used in war. I think most people today have some acceptance of the common ways that wars are waged: with bullets and bombs.

But as we've seen historically, chemical weapons are inhumane and a horrific way to die—and that's why the most of the world has outlawed them.

Another reason is that chemical weapons are indiscriminate—not that bombs and bullets can't be. These weapons kill everyone in any environment where they're deployed. That is why we see the large number of children [killed] in the footage from Syria.

What makes this latest attack different from the chemical attacks allegedly committed by the Syrian government over the last year?

This attack was much larger in scale, and it was more definitive that chemical weapons were used. The previous incidents of chemical weapons use in Syria were not as pronounced and as declarative as this attack. There were many fewer casualties, it wasn't such a widespread area, and there was substantial debate on whether chemical weapons were even involved.

Since then, the U.S. administration and other countries confirmed that yes, chemical weapons had previously been deployed. But they were used on a rather small scale and did not generate quite the response from the international community that this larger dispersion did.

Let's face it, whenever one sees children dying like that, it invokes a much different response from people and countries than a very small-scale incident where just rebel fighters die.

What chemical weapons are suspected of being used during the latest attack, and what are the symptoms in people exposed to them?

Most of the evidence is pointing toward nerve agents, and particularly sarin. Syria has a fairly well-known offensive chemical weapons program, and just based on reports and pictures from the news media and from the symptoms that the patients are exhibiting, sarin gas is the most likely culprit.

A nerve agent like sarin affects the ability of the nervous system to transmit signals to different parts of the body, by blocking the enzyme that the body uses to break down neurotransmitters. What results is overstimulation, and eventually the body may shut down because it's not able to do what it normally can do.

The symptoms are the product of this enzyme being inhibited. Patients may have pinpoint pupils and complain of difficulty seeing or may have a runny nose, which again are the effects of the neurotransmitter being blocked. You may see some twitching, and then eventually they may suffocate because they aren't able to adequately take a breath.

Are there ways to treat these symptoms?

Well, the best way to treat them is prevention. But once a person is exposed, there are a couple of medical antidotes, called atropine and pralidoxime, that can counteract the chemical agent's effects.

Equally important is the decontamination of patients. You can use water, to which you can add some bleach to make it a little bit alkalotic, which will break down the chemicals. In large exposures like this, treatment is very time-dependent, both in administering the antidote as quickly as possible and in decontaminating the patient as much as you can.

Unfortunately, places like Syria sometimes do not have large stockpiles of the antidote and widespread knowledge of decontamination procedures. So the chemical will stick around on the patients' skin, on the clothes.

The chemical is very volatile and it can affect other people around the patients, those taking care of them. So if there are reports of people who are affected by the chemical but who weren't near the scene of the attack, they may be feeling the effects of the chemical that was on patients' clothes and skin.

What are the UN investigators on the ground in Syria looking for?

These experts will most likely look at the different attack scenes and interview the victims, the medical caretakers, and people in the area. They will also be collecting samples and taking them back to the lab.

The main chemical agents degrade very quickly and disappear into the environment, but investigators may find remnants—chemical breakdown products—in human blood, urine, animal carcasses, and soil. Finding the specific breakdown products is almost conclusive evidence that a chemical weapon was used, since the only place where these chemicals could have come from is the breakdown of sarin gas.

It may take a fair amount of time, possibly weeks, before they have the answers. In some cases, patients who are known to have been exposed may not have the chemical in their blood (we saw that happen in the Tokyo subway attack), but in some cases they will. Nothing is really cut and dry when it comes to medical science, so the investigators will be taking a large volume of samples, and they have a lot of work to do.

How does Assad's use of chemical weapons differ from when Iraq's Saddam Hussein used chemical weapons against the Kurds and against Iran in 1988?

It certainly carries a lot of similarities to the attack on the Kurdish citizens of Halabja during the Iran-Iraq war. If you pull up images of Halabja from when Saddam used chemical weapons and look at pictures of the victims in Syria, you'll notice that they are virtually indistinguishable.

With their horrific toll, and with the risk of drawing ire from the international community so high, why use these weapons?

It's hard to figure out what the logic was behind doing something like this with the understanding that the entire world is watching this unfold. One can only make deductions as to what was going on inside the minds of people responsible for this attack. But what would drive someone to use a weapon such as this?

As the U.S. launched operation Iraqi Freedom, military minds were concerned that backed into a corner, Saddam Hussein might use chemical weapons as the last-ditch effort, and we were fortunate that this did not occur. [Iraq turned out to lack a chemical weapons stockpile.]