In the wake of the London bombings, riders of Washington's Metro should know to be on the alert for unattended packages, boxes, bags, luggage or backpacks on trains or train platforms. But there's more:

* Small unattended objects can be dangerous too -- even something as small as a cell phone. If you see such an object, alert Metro authorities. Do not handle it yourself.

* In the case of suicide bombers, riders should be alert for people with unusually bulky clothing or coats worn strangely or inappropriately for the weather.

Riders should also be alert to unusual odors. For example, some explosives have the faint smell of almonds. But more likely, an unusual odor may indicate a chemical attack:

* Some dangerous chemicals have distinct aromas: Cyanide gas has the odor of burnt almonds; mustard gas or blistering agents may smell like mustard, garlic or onions. This is not the unpleasant odor of one person -- it would be pervasive and distinct.

* Sarin, the deadly agent used on the Tokyo subway in 1995, doesn't have an odor, but in Tokyo it was diluted so victims reported an unusual odor at the time.

* Be alert for liquids on the floors of cars or on seats, and any leakage of liquid from passenger baggage, backpacks or containers of any kind.

In case of a chemical attack, people probably will start showing symptoms immediately:

* If you see several people in the same general area suddenly start violently coughing, gagging, fainting, vomiting or going into convulsions, you should report it immediately and move away from the affected area if possible. If your eyes start to burn or your vision gets blurry, the best thing you can do is wash your eyes with water for 10 or 15 minutes. It might make sense to carry a water bottle with you on your daily commute.

* Most people will instinctively try to help victims. Unfortunately, if you handle affected people you could get the chemical on yourself. Again, the best response is to alert the authorities and if possible get to fresh air.

* If a chemical gets on you or your clothing, you should wash completely as soon as possible and any affected clothing should be removed, put in plastic bags and discarded under the direction of authorities. In Tokyo, sarin-saturated clothing continued to inflict injuries long after the initial attack.

Biological attack is another danger:

* Biological agents are usually powders, so be alert to the presence of any unusual powders in subway cars, on the seats, floors or platforms. If you see someone spreading or blowing powder or you see powder leaking from luggage or backpacks, alert authorities.

Fortunately, Washingtonians have one of the best and safest mass transit systems in the world. Because our Metro system is clean and simple, there is little debris or garbage that could hide bombs. The prohibitions against eating and littering provide safety by making discarded or unattended objects stand out.

Metro stations are large and well-lighted and ventilated. Large objects that could hide bombs, such as garbage cans or recycling bins, have been removed or replaced by large, blast-resistant types. As new technologies that can detect chemical, biological or radiological agents arise, they are often installed here first.

It is unfortunate that the extremists' war against us requires this kind of vigilance. But using common sense, staying calm, following the rules, assisting authorities and, most of all, staying alert can make a difference and keep us all safe.

-- David Silverberg

is editor of HSToday, an independent, monthly magazine of homeland security analysis published in McLean.