I went back to messages with my allergy md :
Your postnasal drip and phlegm (which can also contribute to cough) can also be from another condition called vasomotor or nonallergic rhinitis. This condition presents clinically like allergies, but is due to a different mechanism and is more difficult to treat. Please see below for more detailed and helpful information.

There may be triggers in the environment that may cause your symptoms to worsen from time to time. One can be sensitive to factors other than to mites, animals, molds and pollens.

Non-Allergic Factors and Irritants:

Avoid strong odors and other irritants such as:
Tobacco smoke - do not allow smoking in the house or the car
Wood smoke - includes fireplace, structure fires, brush fires
Strong perfumes, talcum powder, hair sprays, cleaning products, new carpets, cooking odors
Paint fumes
Dusts - sawdust, chalk dust, house dust which can contain dirt, hair, pet dander, etc
Sprays - air freshener, insect sprays
Hay
Avoid strong smelling plants and trees indoors.

Changes in temperature or humidity can provoke symptoms. For example, cold temperatures sometimes aggravate nose and chest symptoms.

Air pollution and smog:
Stay inside with the windows closed.
In the car, keep the windows up and use air conditioning

Physical factors
Exercise
Eating (Gustatory)
Cold exposure

Colds & Infections
Avoid Contact with people who have colds or respiratory infections
Wash hands after you are exposed to people who are sick
Yearly flu shots are recommended
Consider the pneumovax vaccination (especially for people who are over 50 years old

VASOMOTOR RHINITIS

If it isn't an allergy, what is it?

Nasal congestion, runny nose, postnasal drip, and sinus headache are signs of a sensitive and inflamed nose (rhinitis). These symptoms can indicate two common conditions, allergic rhinits or vasomotor rhinitis. The symptoms for both conditions are very similar. However, the substances that cause them, and the way we test for and treat them, are very different.

When an immune reaction in the nose is causing the symptoms, the condition is called allergic rhinitis, and the offending substance is called an allergen. Common allergens include pollen, animal dander, and dust mites. Itching is usually one of the most bothersome symptom of allergic rhinits.

When a nerve ending reaction in the nose is causing the symptoms, the condition is called vasomotor rhinits. In this case, the offending substance is called an irritant. Common irritants include smoke, perfumes, weather changes, and any strong chemical fumes or odors. (Even the aroma from certain flowers and trees can act as irritants.) Nasal congestion, sinus headache, and postnasal drip are usually the most bothersome symptoms of vasomotor rhinitis.

Why don't the tests show reactions?

Allergy tests only detect immune reactions (by skin test or blood test). Because there is no immune reaction involved in vasomotor rhinitis, tests can't identify the troublesome irritants. People usually know the obvious irritants well from personal experience!

How does a person develop this condition?

We don't know. Allergic rhinitis tends to run in families and typically starts in childhood or adolescence. Vasomotor rhinitis doesn't usually run in families and can start at any age. Often, there was a 'cold that never went away' or the condition developed after an intense exposure to a strong irritant. But most of the time, symptoms developed 'out of the blue'.

How long will it last?

Vasomotor rhinitis does not follow a regular pattern. It may go away quickly, or it can last for months or years.

What can be done for vasomotor rhinitis?

There are several ways to help control vasomotor rhinits.

First, avoid obvious irritants when possible. Cigarette smoke, perfumes, strong detergents, and other chemical fumes can often be avoided especially in the home. Other irritants (such as smog, car fumes, and weather changes) are nearly impossible to avoid.

Secondly, irrigate your nose and sinuses with salt water on a regular basis. Add a quarter teaspoon of salt and a quarter teaspoon of baking soda to 8 ounces of lukewarm water. Flush this solution in the nose with a bulb syringe or nasal rinse bottle 2-3 times a day. (Making a harsh "K" sound at the same time will prevent water from draining down the back of the throat.) Many people find this method an effective way to control symptoms.

Finally, most people with vasomotor rhinits need to take medicines regularly. The most effective medicine for most people is a nasal corticosteroid such as fluticasone (flonase) or flunisolide (nasarel). Using this type of medication daily can greatly reduce sinus sensitivity, and decrease sinus congestion and postnasal drip. Decongestant pills such as Sudafed have the advantage that they can work on an as needed basis. If post-nasal drip or runny nose are troublesome, other nasal sprays called azelastine or ipratropium, can be added.

What about allergy shots or other treatments?

Allergy shots work by changing the immune reaction which is the basis of an allergy. Because the irritants that cause vasomotor rhinitis do not trigger immune reactions, allergy shots won't help.

Here is some information about rhinitis from the Academy of Allergy, Asthma and Immunology:
http://aaaai.execinc.com/videos/conditions-and-treatments/allergy-videos/what-is-rhinitis.asp