Thresholds for effect

End point/Concentration/Rationale The highest no-effect concentration of 8,000 ppm for 1 h was used as the basis for the AEGL-1. This concentration is considerably below the threshold for effects in animal studies. For example, anesthetic effects occur at a concentration of approximately 200,000 ppm. 

Approaches to duration adjustment are reviewed in Kimmel et al. (2006). Prior to derivation of NOAELs, LOAELs, or BMDs/ BMCs, the toxicity data are adjusted to a continuous exposure scenario. For oral studies, a daily exposure adjustment is made (e.g. a five days per week exposure is converted to seven days per week). For inhalation exposures, a concentration x time (c x t) adjustment is made. Traditionally, the inhalation exposure adjustment has not been done, because of concerns about peak versus integrated exposure and the likelihood of a threshold for effects. However, a review of the RfD and RfC processes by the USEPA recommended that inhalation developmental toxicity studies be adjusted in the same way as for other end-points (USEPA, 2002b). Derivation of a human equivalent concentration for inhalation exposures is intended to account for pharmacokinetic differences between humans and animals. 

Being able to predict the human health risk from exposure to airborne chemicals can be complex, requiring reliable analysis of human exposure. While the basic principles of risk assessment are applicable to various conditions of exposure, characterizing how an individual s health status can significantly influence the threshold for effects can be a most challenging component of the risk assessment process. One needs to consider the overall scientific weight-of-evidence to predict whether or not an individual may be uniquely susceptible to certain... 

Because so many factors in addition to lipid composition can affect POP accumulation (discussed above), some researchers currently favor using whole body lipid and POP measurements to estimate risk of effects from POP exposure. This is essentially a lipid-adjusted CBR. A recent example of this approach is shown in detail by Meador et al.122. These authors used a number of studies from the literature on appropriate life stages of salmon to determine a lipid-adjusted threshold for effects for a wide range of endpoints after PCB exposure. This threshold number was then compared to levels of PCBs measured in wild juvenile salmon from a contaminated estuary, and many of the fish, on the lipid-adjusted basis, were determined to be at risk of effects from environmental exposure to PCBs. 

The possibility cannot be excluded that specific VOCs appear indoors which may turn out in the future to he much more potent in causing effects on humans than the average VOCs. In this case, these (e.g. formaldehyde) should be evaluated individually, and a list of such compounds should be established. A draft list of special compounds for which low thresholds for effects or high detection limits cause the probability of sensory effects to be grossly underestimated if this TVOC-concept is used has not yet been officially established. This list should also include compounds which are likely to cause other types of health effects than irritation and which, therefore, must be considered in a total evaluation of the indoor air quality. Potential candidates for the list are mentioned above. 

For compounds with genotoxic effects, one assumes that there is no safe lower level of exposure, even though the risk may be very low. The term as low as reasonably achievable (ALARA) is often used for toxins without a threshold for effect. In such cases, estimates are made of exposures that constitute a risk of cancer in 1/10 or 1/10 at hfelong exposure. Instead, one may use the principle of margin of exposure (MOE) for such compounds, which means calculation of the ratio between the NOAEL and the estimated exposure. 

Data regarding effects on the fertility of rats and mice are limited by study design. Dose levels were not chosen to detect thresholds for effects on fertility, and the studies were not designed to investigate the mechanisms by which chlordane may interfere with fertility. The data in animals are not sufficient to indicate whether chlordane can cause reproductive effects in humans, or whether such effects would be expected in humans exposed to chlordane near waste sites. 

The threshold for effective down-conversion to commence in an SRO is comparatively high, and this is often difficult to reach when pumping with a CW laser source. In order to increase the conversion efficiency, the OPO cavity can be made resonant at both signal and idler frequencies, forming a so-called doubly... 

Most common model used by the U.S. ERA to develop slope factors assumes a linecu dose-response relationship and no threshold for effects. 

Blood lead concentration (PbB) and erythrocytic protoporphyrin concentration (EP) are both used as indices of lead exposure in children and in occupationally exposed adults. In a previous study we reported dose-effect and dose-response relationships, in EP vs. PbB (Hammond et al, 1955). In the present study we report the influence of age on the threshold for an EP response to PbB, as well as on the slope of the dose-effect interaction. In the age range 12-30 months the threshold for effect of PbB on EP falls progressively, while the magnitude of the dose-effect increases with age. The rate of decline in the threshold for In EP and the rate of increase for In EP PbB are linearly related to age. 

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