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Biomedical subjects

P Mehnert

Publications and source records attributed to P Mehnert.

11 recordsLinked to original sources
PubMed2002

Assessment of the risk of heat disorders encountered during work in hot conditions.

OBJECTIVE: To co-ordinate the work of the main European research teams in the field of thermal factors in order to develop and improve significantly the methods presently available for assessing the risks of heat disorders encountered during work in hot conditions. METHOD: Each item from the required sweat rate model was reviewed on the basis of the most recent literature. A database with 1,113 laboratory and field experiments, covering the whole range of hot working conditions, was assembled and used for the validation. RESULTS: Influence of clothing ensemble on heat exchange: methods and formulas were developed that take into account the dynamic effects associated with forced convection and the pumping effect associated with body movements and exercise. Prediction of the average skin temperature: the model used in the required sweat rate standard ISO 7933 was extended to cover more severe conditions with high radiation and high humidity and different clothing and take into account the rectal temperature for the prediction of the skin temperature. Criteria for estimating acceptable exposure times in hot work environments: criteria were reviewed and updated concerning the maximum increase in core temperature and the acceptable water loss, for acclimatised and nonacclimatised subjects. These limits are intended to protect 95% of the population. Measuring strategy: a strategy was developed to assess the risks in any working situation with varying conditions of climate, metabolic rate or clothing. A detailed methodology was developed in three stages: an "observation" method for the recognition of the conditions that might lead to thermal stress; an "analysis" method for evaluating the problem and optimising the solutions; and an "expert" method for in-depth analysis of the working situation when needed. VALIDATION: the different results were used to prepare a revision of the interpretation procedure proposed in the ISO standard 7933. We validated the modified approaches using the database. This involved the whole range of conditions for which the model was extended, namely conditions with high and low radiation, humidity and air velocity as well as fluctuating conditions. Based on these results, the predicted heat strain model was developed: it is presently proposed as an ISO and CEN standard.

Biomechanical Phenomena↗
PubMed2001

Experiments on the effects of a continuous 16.7 Hz magnetic field on melatonin secretion, core body temperature, and heart rates in humans.

The present study investigated the hypothesis that a strong extremely low frequency magnetic field partially suppresses the synthesis of melatonin and subsequently elevates the core body temperature. Seven healthy young men (16-22 years) took part in a control and in an exposure session. Three men experienced first the control and then the exposure session, four men experienced the sessions in reverse order. Control sessions were performed as constant routines, where the participants spent 24 hour periods continuously in bed while air temperature was 18 degrees C, illumination less than 30 lux, and the sound pressure level 50 dBA. The exposure sessions differed from that protocol only between 6 pm and 2 am when a strong extremely low frequency magnetic field was continuously applied (16.7 Hz, 0.2 mT). Assuming that the participants were unable to perceive the field consciously, they were blind against the actual condition. Salivary melatonin levels were determined hourly; body core temperatures and heart rates were registered continuously throughout. Neither of these parameters revealed alterations that can be related to the influence of the magnetic field. The present results, taken together with other investigations using that particular field, lead to the hypothesis that the effects most likely, occur, only after repetitive exposures to intermittent fields.

Adolescent↗
PubMed2001

Comparison of laboratory studies with predictions of the required sweat rate index (ISO 7933) for climates with moderate to high thermal radiation.

For International Standard ISO 7933 (Hot environments-analytical determination and interpretation of thermal stress using calculation of required sweat rate. International Organization for Standardization, Geneva, 1989) it has been questioned whether the heat stress index "required sweat rate" is applicable in environments where mean radiant temperature (t(r)) exceeds dry air temperature (t(a)). Based on a re-analysis of 556 climatic chamber experiments on 16 subjects the observed sweat rates were compared with required sweat rates predicted according to the ISO standard. Under climatic conditions with t(r) > t(a) the predictions overestimate the observed sweat rates on average by 86% in clothed subjects and by 25% in nude subjects. In climates with t(r) = t(a) the predictions highly agreed with the observed sweat rates in nude subjects and were slightly overestimated (13%) in clothed subjects. The misjudgement of the present ISO standard is obviously caused by an inappropriate calculation of radiative heat exchange. Corrections are proposed to improve the validity concerning climates with t(r) > t(a).

Adult↗
PubMed2001

Development and validation of the predicted heat strain model.

Eight laboratories participated in a concerted research project on the assessment of hot working conditions. The objectives were, among others, to co-ordinate the work of the main European research teams in the field of thermal factors and to improve the methods available to assess the risks of heat disorders at the workplace, and in particular the "Required Sweat Rate" model as presented in International Standard ISO 7933 Standard (1989). The scientific bases of this standard were thoroughly reviewed and a revised model, called "Predicted Heat Strain" (PHS), was developed. This model was then used to predict the minute by minute sweat rates and rectal temperatures during 909 laboratory and field experiments collected from the partners. The Pearson correlation coefficients between observed and predicted values were equal to 0.76 and 0.66 for laboratory experiments and 0.74 and 0.59 for field experiments, respectively, for the sweat rates and the rectal temperatures. The change in sweat rate with time was predicted more accurately by the PHS model than by the required sweat rate model. This suggests that the PHS model would provide an improved basis upon which to determine allowable exposure times from the predicted heat strain in terms of dehydration and increased core temperature.

Algorithms↗
PubMed2000

Criteria for estimating acceptable exposure times in hot working environments: a review.

OBJECTIVES: To revise the criteria used in the present "Required Sweat Rate" standard ISO 7933 (1989) for the prediction of the maximum duration of work in hot environments. METHODS: Review of the literature and in particular, of the bases for the present criteria. RESULTS: A new method is proposed, to take into account the increase in core temperature associated with activity in neutral environments. The prediction of maximum wetness and maximum sweat rates are revised, as well as the limits for maximum water loss and core temperature. CONCLUSION: An improved set of maximum values and limits is described, to be used in the revised version of the ISO 7933 standard. Due to the major modifications to the "Required Sweat Rate" index and in order to avoid any confusion, it is suggested that the revised model be renamed the "Predicted Heat Strain" (PHS) model.

Ergonomics↗
PubMed2000

Prediction of the average skin temperature in warm and hot environments.

The prediction of the mean skin temperature used for the Required Sweat Rate index was criticised for not being valid in conditions with high radiation and high humidity. Based on a large database provided by 9 institutes, 1999 data points obtained using steady-state conditions, from 1399 experiments and involving 377 male subjects, were used for the development of a new prediction model. The observed mean skin temperatures ranged from 30.7 degrees C to 38.6 degrees C. Experimental conditions included air temperatures (Ta) between 20 and 55 degrees C, mean radiant temperatures (Tr) up to 145 degrees C, partial vapour pressures (Pa) from 0.2 to 5.3 kPa, air velocities (v(a)) between 0.1 and 2 m/s, and metabolic rates (M) from 102 to 620 W. Rectal temperature (T(re)) was included in the models to increase the accuracy of prediction. Separate models were derived for nude (clothing insulation, I(cl), < or = 0.2 clo, where 1 clo = 0.155 m2 x degrees C x W(-1), which is equivalent to the thermal insulation of clothing necessary to maintain a resting subject in comfort in a normally ventilated room, air movement = 10 cm/s, at a temperature of 21 degrees C and a humidity of less than 50%) and clothed (0.6 < or = I(cl) < or = 1.0 clo) subjects using a multiple linear regression technique with re-sampling (non-parametric bootstrap). The following expressions were obtained for nude and clothed subjects, respectively: T(sk) = 7.19 + 0.064Ta + 0.061Tr + 0.198Pa - 0.348v(a) + 0.616T(re) and T(sk) = 12.17 + 0.020Ta + 0.044Tr + 0.194Pa - 0.253v(a) + 0.0029M + 0.513T(re). For the nude and clothed subjects, 83.3% and 81.8%, respectively, of the predicted skin temperatures were within the range of +/- 1 degree C of the observed skin temperatures. It is concluded that the proposed models for the prediction of the mean skin temperature are valid for a wide range of warm and hot ambient conditions in steady-state conditions, including those of high radiation and high humidity.

Clothing↗
PubMed2000

Drafts in cold environments--the significance of air temperature and direction.

This paper concerns the influence of air temperature and of the direction of drafts on subjective and physiological responses. In three experimental series 58 healthy persons (50 men, 8 women, 20-29 yrs) were exposed to drafts in overall 446 experimental sessions. Drafts were applied either horizontally or vertically with mean air velocities of 0.1 to 0.3 m/s and a turbulence intensity of 50%. Air temperature was varied between 11 and 23 degrees C and metabolic rates between < 70 and 156 W/m2. These parameters were kept constant during the single one-hour sessions. The subjects were dressed for thermal neutrality. Draft-induced annoyance was registered every 5 minutes using a list of prescribed body parts and skin temperature was measured at the forearm and at the neck. Subjective and physiological responses were systematically related to air temperature. Draft-induced general annoyance, draft-induced local annoyance (neck, forearm) and the drop of the corresponding skin temperature were inversely related to air temperature. Concerning the direction horizontal drafts seem to cause somewhat stronger reactions. The predictive model developed by Toftum underestimates the percentage of persons annoyed. A modified version increases the predictive power significantly.

Adult↗
PubMed1991

[Treatment of Monteggia fracture in children].

Described in this paper is a therapeutic concept by which to cope with Monteggia's fracture in childhood. It is based on experience obtained from 72 cases with injuries of that kind and their evaluation in a group study. Therapeutic approach to the individual case was determined by the following criteria: age of infant, localisation and shape of ulnar fracture, reducibility of capitulum radii, and accompanying injuries requiring treatment on the same arm. The need for surgical stabilisation of ulnar fracture was found to increase along with growing age of the affected child. High-stability anatomic reduction of the ulnar fracture proved to be a prerequisite for safe stabilisation of the radial capitulum. Close reduction has proved to be sufficient in many instances. Open reduction and internal fixation were found to be necessary in cases in which an ulnar fracture was irreducible or instable and/or a radial head was not reducible. Minimal osteosynthesis and plaster cast is considered to be an optional therapy for younger children, whereas ulnar stabilisation by means of plates is preferred for children in somewhat advanced age of childhood. The radial head may be fixed by trans-articular Kirschner's wire (WITT) or primary reconstruction of the annular ligament, using a strip of biceps or triceps tendon, or adaptation around the collum radii and ulna of the proximal radio-ulnar joint by means of a sling of Dexon or Vicryl suture.

Child↗
PubMed1990

[The surgical realization of optimal timing for undescended testicle].

An account is given in this paper of surgical indications to cope with cryptorchism. Views on timing of surgical action have undergone considerable change over the past 35 years, ranging from prepuberty to the second year of age, the latter date being considered optimal now. Optimal timing is not kept in practice, as may be seen from a regional longitudinal study over 25 years as well as from a representative inquiry among 17 centres of paediatric surgery across the GDR over the same period of time and from a cross-section analysis of all boys operated on for cryptorchism throughout the GDR in 1986. All medical doctors serving at infant welfare centres and day nurseries are invited to refer affected boys to surgeons at an earlier date. The existing health system for children and adolescents is considered to provide good conditions for lowering the average age at surgery.

Age Factors↗