Guidelines for Dengue Surveillance and Mosquito Control, 1995 (WHO  OMS, 1995, 112 p.) 
ANNEXES 
The steps described below mainly refer to treatment of mosquito nets with permethrin. The net treatment technique can be easily used for curtains.
a. Calculate the area to be treated.
Measure the height, length and width of net. Assuming a rectangular mosquito net is 150 cm high, 200 cm long and 107 cm wide, the calculations are as follows:
Area of one end 
= 
107x150 
= 
16 050 cm^{2} 
Area of one side 
= 
200x150 
= 
30 000 cm^{2} 
Area of to 
= 
107x200 
= 
21 400 cm^{2} 
The sides and ends need to be multiplied by 2.
2(16 050 + 
30 000) = 92 100 + 
21 400 = 113 500 cm^{2} 
(end) 
(side) 
(top) 
If 10 000 cm^{2} = 1m^{2} then
113 500/10 000 = 11.35 m^{2} area of net
Assume a permethrin emulsifiable concentrate will be used, and the dosage desired is 0.5 grams per square metre.
To determine the total grams required, multiply net size by dosage:
11.35 x 0.11.35 x 0.5 
= 
5.67 grams of insecticide needed 
c. Determine the amount of liquid to saturate a net.
To determine the percentage solution to be used by dipping, it is first necessary to determine the aproximate amount of water retained by a net. Another term for dipping is soaking.
Pour five litres of water, but preferably a dilute solution of the insecticide to be used, into a plastic pan or other suitable container. For cotton, a 0.3% solution can be tried, for polyethylene or other synthetic fiber, a 1.5% solution can be tried. Add one net to thoroughly wet and remove. Allow drips to fall into a bucket for 15 to 30 seconds. Set net aside. Repeat process with two other nets. Cotton nets can be lightly squeezed but not the synthetic ones. Measure water or solution remaining in dripping/soaking container and in the bucket to calculate the amount of liquid used per net.
Assuming that one polyethylene net retained 280 ml of solution, the % concentration required for dipping is calculated as follows:
_{}
d. Preparation of dipping solutions to treat bulk quantities of mosquito nets or curtains.
The general formula is:
X = (A/B)  1
in which X = parts of water to be added to 1 part of emulsifiable concentrateA = concentration of the emulsifiable concentrate (%)
B = required concentration of the final solution (%)
Example: A 2.0% solution of permethrin for dipping nylon mosquito nets or curtains is to be prepared from a 25% concentrate.
X = (25/2.0)  1 =
12.5  1 = 11.5
Therefore 11.5 parts of water to 1 part of concentrate are required, or one litre of concentrate to 11.5 litres of water.
Example: A 2.0% solution of permethrin for dipping nylon mosquito nets or curtains is to be prepared from a 50% concentrate.
X = (50/2)  1 = 24
Therefore, 24 parts of water to 1 part of concentrate are required, or one litre of concentrate to 24 litres of water.
Example: A 0.3% solution of permethrin for dipping cotton mosquito nets or curtains is to be prepared from a 25% concentrate.
X = (25/.3)  1 =
83.3  1 = 82.3 rounded to 82.
Therefore, 82 parts of water to 1 part concentrate are required, or one litre of concentrate to 82 litres of water, or onehalf litre of concentrate to 41 litres of water to accommodate a smaller container.
Example: A 0.3% solution of permethrin for dipping cotton mosquito nets or curtains is to be prepared from a 50% concentrate.
X = (50/.3)  1 =
166.6  1 = 165.6 or rounded to 166.
Therefore 166 parts of water to 1 part of concentrate are required, or one litre of concentrate to 166 litres of water, or one half litre of concentrate to 83 litre of water to accumulate a smaller container,
For 25% concentrate
Add 11.5 litres water to container (with premeasured marks to indicate volume)
Add 1 litre (1 bottle) concentrate to container

Total volume: 
12.5 litres 

Grams permethrin: 
250 

%concentration: 
2% 
For 50% concentrate
Add 24 litres water to container
Add 1 litre (1 bottle) concentrate to container

Total volume: 
25 litres 

Grams permethrin: 
500 

% concentration: 
2% 
f. Preparing a 0.3% dipping solution using a one litre bottle of 25% or 50% permethrin emulsifiable concentrate for soaking cotton nets or curtains.
For 25% concentrate
Add 82 litres of water to container
Add 1 litre (1 bottle) concentrate to container

Total volume: 
83 litres 

Grams permethrin: 
250 litres 

% concentration: 
0.3% 
For 50% concentrate
Add 166 litre of water to container
Add 1 litre (1 bottle) concentrate to containers

Total volume: 
167 litres 

Grams permethrin: 
500 

% concentration: 
0.3% 
g. Drying of nets
Polyethylene and synthetic nets are dried in a horizontal position. Do not hang to dry. Drying the nets on mats removed from houses has proved to be convenient and acceptable. The nets are turned over about once every hour up to 3 to 4 hours. If the weather is good, the nets can be dried outside in the sun but not more than several hours. Under rainy conditions, they can be placed under sheltered areas or inside and left overnight to dry. When dripping no longer occurs, they can be hung up to dry. Treated cotton nets which are not over saturated and do not drip can be hung up to dry soon after the soaking procedure.
As shown in (a) above, it is assumed that net size is 11.35 m^{2}, 5.67 grams of permethrin are needed to achieve a target dosage of 0.5 grams per square metre, and this size net absorbs 280 ml of solution.The amount of 25% permethrin emulsifiable concentrate to use is determined as follows:
_{}
Therefore, 23 ml of 25% permethrin is mixed with 280 ml of water. The net is placed inside the bag and the solution added. The net and solution are mixed together, shaken and kneaded in the bag. The net is removed and dried on top of the bag or a mat as described in (g) above. The amount of water can be reduced by 23 ml if there is excess run off after the net is removed from the bag.
Important points in the treatment are summarized as follows:
1. Dipping is the preferred method of net treatment. A 2% solution is usually sufficient to achieve a target dosage of 0.5 grams per square meter of permethrin on polyethylene, polyester, nylon or other type of synthetic fiber net or curtain. The residual effect lasts for 6 months or more. A 2% solution can be simply prepared by pouring the contents of a onelitre bottle of 25% permethrin emulsion concentrate into a container with 11.5 litres of water. With a 50% concentrate, one litre is poured into 24 litres of water. The container used can be marked to show one or both of these volume levels. A 0.3% solution is normally required for cotton material, which absorbs more liquid. Responsible staff need to check on the dosage applied and refine the operation accordingly. With bamboo curtains or mats used over doors or windows, a higher dosage (1.0 grams per square meter) can be used.2. Dipping the nets in a permethrin solution is a fast and simple method for treating nets and curtains under urban or rural housing conditions. Community members rapidly learn the technique which is required for followup treatment A dishpan type of plastic or aluminum container which holds 15 to 25 liters of solution has been found to be quite suitable. Normally, about one liter of solution can treat about 4 to 5 double (10m2) size polyethylene or polyester nets. When the nets are removed from the solution, they should be held to drip in a bucket for no more than one minute before being laid out to dry in a horizontal position. Straw mats removed from houses are quite suitable for drying the nets outside in open air. With one dipping station, about 150 nets or curtains can be treated in two hours or less.
3. One can assume that 100 treated double size nets or an equivalent area of curtain material can protect 250 persons. It is not reasonable to expect every person in a crowded household to sleep under a net. It is important that every house in a community or village has one or two treated nets to kill mosquitos to reduce the vector density. When used in this manner, protection is provided to those who do not even sleep under the nets. Infants and small children can sleep under the nets during day time.