Toxicity

With respect to the hazards of a chemical spills the term “toxicity” is often used. This paragraph will describe in more detail what the term toxicity means and how it should be interpreted in the various situations.

Each substance has a different value for its various toxicity thresholds. These toxicity threshold values have to be compared to the on scene concentration in case of an emergency. Toxicity is a property of a substance involved.

In fact toxicity is quite complex it depend on:

(1) The substance involved,

(2) The exposure time,

(3) The % of organism killed if exposed during a given exposure time,

(4) The type of organisms,

(5) ) The compartment of exposure

(6)Type of effect it will have and

1 Substance involved

First of all the term toxicity is a property of a certain chemical like the LC50(96) value of a substance X. LC stands for Lethal Concentration. 50 stands for the percentage of organism killed if exposed to this concentration while the term (96) is the hours of exposure to arrive at this killing percentage.

2 The exposure time

The exposure time is the time an organism comes into contact with the substance at a certain concentration. The relevant exposure time depends on the compartment the substance will go.

For the air the exposure is often relatively short. Due to the wind speed a substance will transported very fast in the air. As an example at Beaufort 5 (10 m/sec wind) a cloud will travel 36 km per hour. There for the exposure time is often short 10, 20 minutes in emergency situation. In factories where people work every day of the week with a particular chemical longer exposure times are used to describe the no effect concentration.

Assuming a spill plume of 100 kg (100.000.000 mg) with a concentration of 1 mg/m3, 100.000.000 m3 air will be polluted. Assuming a height of the cloud of 25 m and a width of 100 m this will result in a length of the cloud of 40 km. The same spill but now with a concentration of 10 mg/m3 and the same height and width will have a length of 4 km and for a 100 mg/m3 spill the length will be only 400 m

At Beaufort 5 (10 m/sec) the exposure times will respectively be 66 minutes, 6,6 minutes and 0,6 minutes just to give you an indication how short the exposure time will be in the air compartment.

In the water compartment the substances will dilute but some marine organisms exposed will stay in the plume as they move with the water like the plume will do. Therefore the exposure will be much longer and the situation will be save for those organisms as the plume in the water is diluted till a No Effect Concentration (NEC).

The longer the exposure time the more effects will happen.

3 The % of organisms killed

As already said before the percentage killed could vary. The longer the exposure time the more organisms will be killed. In case of an emergency sometimes a threshold value will be accepted where 5% of the most sensitive organisms will be killed for a short exposure time. For instance in case of using dispersants it is accepted that a concentration of 10 mg/l is acceptable for oil spills which is the LC5(4) Lethal concentration for 5% of a sensitive organism during a exposure time of 4 hours. As this is an emergency situation and using dispersants will bring the substance into the water column one has to accept some limited damage.

4 The type of organism

Each type of organism has its own threshold toxicity values at various exposure times. The type of organisms exposed depend on the compartment where the substance will go after release: If the substance goes into the air it could be ducks, birds or even human beings.

If the substance goes into the water column any marine organism need to be considered. Scrimps, fish etc. they all have a different toxicity threshold value for a particular chemical. Often more toxicity threshold values for different organism will be considered. The most sensitive organisms often will be used.

5 The compartment of exposure

Above toxicity values are for exposure to the chemical in the different compartments. Organisms including human beings could swallow a substance too.

A distinction can be made between the compartments a substance will go e.g.:

1. Exposure into the air - expressed in mg/m3,
2. Exposure into the water column - expressed in mg/l
3. In the body itself - expressed in mg/ kg body weight

(6) The type of effect of an exposure: the effect will be that 50% of the organisms will be killed if exposed for 96 hours to that concentration (Lethal Concentration) for a substance with a LC50(96) Exposed to a lower concentration less organisms will be killed. Four different types of effects could be distinguished: (1) effect is deadly, (2) reversible effect, (3) irreversible effect and (4) no effect. In case of other than lethal effects these should be described such as fish will swim to the surface etc.

The following table gives an overview of different threshold values and the expressions used.

As long as the concentration on scene stays below the no effect threshold value the situation is safe.

The threshold values for the LC50(96) are for instance:

Highly toxic substances less than 1 mg/l

Moderately toxic substances 1 - 10 mg/l

Slightly toxic substances 10 – 100 mg/l

Practically non-toxic substances100 – 1000 mg/l

Non-hazardous substances > 1000 mg/l

As clearly can be seen from this table that the higher the toxicity threshold value the less toxic a substance will be.

If the on scene concentration is 100 mg/l for a substance with a LC50(96) of 1000 mg/l the situation is safe while for a substance with a LC50(96) of 1 mg/l the situation is quite unsafe for the organisms exposed to it for 96 hours.

Table 21
Compartment	Unit	No effect	Reversible effects	Irreversible effects	Killing effects	Exposure times
Air	Mg/m3	MAC	EC	EC	LC50(time)	Minutes
TLV
IDLH
		EPEL
Water column	Mg/l	NOEL	EC50 (time)	EC (time	LC50 96	Hours
NEL			LC50 (4)
0,01*LC50(96)
Oral intake	Mg/kg body weight	ADI			LD (time)	days

No effect concentration thresholds

In emergency situations we accept that the threshold value in the air may be 3 times the no effect concentration value. For human being, the most important organisms in the air compartment, the 3 times the MAC value is still acceptable in emergency situations. The Mac value is the threshold value for human beings working and exposed to that particular substance for 8 hours a day during a 5 days week without any effect.

In the water compartment 1% of the LC50(96) value normally is used to describe the safe situation where no effects take place.

For human beings the no effect concentration in the body will be the ADI value (Acceptable Daily Intake), which normally is used in the food industry.

Another important aspect is that substances will dilute which means that the concentration in the middle of the cloud will be higher than at the border and that the concentration on scene is not constant during the exposure time.